Block Email Spam with Postfix and SpamAssassin Content Filter

In previous articles, we discussed how you can quickly set up your own mail server using iRedMail and  7 effective methods for blocking email spam with Postfix SMTP server. I’m going to share more tips and tricks to block email spam in this article. Specifically, we will see how to check email header and body with Postfix and SpamAssassin (SA) to detect spam. SpamAssassin is a free, open-source, flexible and powerful spam-fighting tool.

Email Header and Body Checks with Postfix SMTP Server

Postfix provides 4 simple content checking parameters.

  • header_checks
  • mime_header_checks
  • nested_header_checks
  • body_checks

Postfix will check all inbound emails when any of the above parameters is being used. Each parameter points to a lookup table containing regular expression patterns and actions. The patterns are compared to strings within email messages (header and body). If Postfix finds a match, the specified action is executed. Header and body checks are done by the Postfix cleanup daemon.

There are mainly two types of regular expressions that can be used by Postfix.

  • regexp: POSIX regular expression
  • PCRE: Perl compatible regular expression

Postfix comes with POSIX regular expression support, but PCRE is way faster. To use PCRE in Postfix, you need to install the postfix-pcre package.

sudo apt install postfix-pcre

Run the following command and you will see pcre is now supported.

postconf -m

postfix pcre perl compatible regular expression

Header Checks

To enable header_checks in Postfix, open the main configuration file.

sudo nano /etc/postfix/main.cf

Add the following line at the end of the file.

header_checks = pcre:/etc/postfix/header_checks

Save and close the file. Then you need to create the /etc/postfix/header_checks lookup file with a command line text editor such as Nano.

sudo nano /etc/postfix/header_checks

You can add regular expression checking like below.

/free mortgage quote/     REJECT
/repair your credit/      REJECT

The lefthand key is a regular expression enclosed by two forward slashes. If any of the strings on the leftland appear in any of the headers of an email message (these would most likely show up in the Subject: header), the message is rejected during the SMTP dialog. By default regular expression checking is not case-sensitive.

You can also use DISCARD, instead of REJECT.

/free mortgage quote/    DISCARD
/repair your credit/     DISCARD

This will cause Postfix to claim successful delivery and silently discard the message. DISCARD makes it look as if the message was delivered even though it was simply thrown away. I often use DISCARD when I don’t want the spammer to know I have blocked a certain phrase for incoming email. DISCARD can also be useful to minimize the backscatter problem. If an innocent user’s email address is used as the sender address, you can claim successful delivery, so that the innocent user does not receive bounce messages.

Some stupid spammers use multiple email addresses in the To: header, instead of using Blind Carbon Copy (BCC). If you are sure an email address won’t be accepting emails with multiple recipients in the To: header, you can add the following lines to discard such email.

/To:.*(gmail.com|yahoo.com|outlook|hotmail.com).*[email protected]/       DISCARD
/To:.*[email protected].*(gmail.com|yahoo.com|outlook|hotmail.com)/       DISCARD

The above lines will check if an Gmail/Yahoo/Outlook/Hotmail address and your domain email address are in the To: header at the same time. If true, the email will be discarded. The two characters .* are a wild card in regular expressions that can be matched to any characters.

Some spammers use blank email address in the From: or To: header, you can add the following checks.

/To:.*<>/           DISCARD
/From:.*<>/         DISCARD

Once you finish editing the header_checks lookup file, you need to build the index file.

sudo postmap /etc/postfix/header_checks

Then restart Postfix for the changes to take effect.

sudo systemctl restart postfix

Body Checks

In addition to header checks, Postfix can check the body of an email message. To enable body_checks in Postfix, open the main configuration file.

sudo nano /etc/postfix/main.cf

Add the following line at the end of the file.

body_checks = pcre:/etc/postfix/body_checks

Save and close the file. Then you need to create the /etc/postfix/body_checks lookup file.

sudo nano /etc/postfix/body_checks

You can add regular expression checking like below.

/free mortgage quote/     REJECT
/repair your credit/      REJECT

You can use DISCARD, instead REJECT.

/free mortgage quote/     DISCARD
/repair your credit/      DISCARD

The patterns indicated by the body_checks parameter are checked against each line of the body of the message. If any of the strings on the leftland appear in the body of an email message, the message is rejected or discarded. Once you finish editing the body_checks lookup file, you need to build the index file.

sudo postmap /etc/postfix/body_checks

Then restart Postfix for the changes to take effect.

sudo systemctl restart postfix

SpamAssassin Content Filter

The built-in content checking in Postfix is very simple. However, there is no way to whitelist individual messages that you might want to receive despite their containing phrases that trigger a rejection and you might not want to reject or discard an email message based on a single rule. For more sophisticated analysis, we need to use a dedicated content filter (such as SpamAssassin) specifically designed to detect spam.

SpamAssassin is a score-based system. It will check email message against a large set of rules, in contrast to a single rule in Postfix. Each rule adds or removes points in the message’s score. If the score is high enough (by default 5.0), the message is considered spam.

Install SpamAssassin

Note: If you used iRedMail to set up your mail server, then SpamAssassin is already installed alongside Amavis, which can read SpamAssassin rules. You don’t need to follow the instructions in this section.

Run the following command to install SpamAssassin from the default Ubuntu software repository. Spamc is the client for SpamAssassin spam filtering daemon.

sudo apt install spamassassin spamc

During the installation, the debian-spamd user and group will be automatically created. The binary installed by the spamassassin package is called spamd, which will be listening on TCP port 783 on local host.

spamassassin content filter

By default, the spamassassin systemd service is disabled, you can enable auto start at boot time with:

sudo systemctl enable spamassassin

Then start SpamAssassin.

sudo systemctl start spamassassin

Integrate SpamAssassin with Postfix SMTP Server as a Milter

There are several ways you can use to integrate SpamAssassin with Postfix. I prefer to use SpamAssassin via the sendmail milter interface, because it allows me to reject an email when it gets a very high score such as 8, so it will never be seen by the recipient.

Install the spamass-filter packages from Ubuntu default software repository.

sudo apt install spamass-milter

Next, edit /etc/postfix/main.cf file and add the following lines at the end of the file.

# Milter configuration
milter_default_action = accept
milter_protocol = 6
smtpd_milters = local:spamass/spamass.sock
non_smtpd_milters = $smtpd_milters

If you have configured OpenDKIM and OpenDMARC, then these lines should look like below. The order matters.

# Milter configuration
milter_default_action = accept
milter_protocol = 6
smtpd_milters = local:opendkim/opendkim.sock,local:opendmarc/opendmarc.sock,local:spamass/spamass.sock
non_smtpd_milters = $smtpd_milters

Save and close the file. Now open the /etc/default/spamass-milter file and find the following line.

#OPTIONS="${OPTIONS} -r 15"

Uncomment this line and change 15 to your preferred reject score such as 8.

OPTIONS="${OPTIONS} -r 8"

If the score of a particular email is over 8, Spamassassin would reject it and you would find a message like below in the /var/log/mail.log file, indicating it’s rejected.

milter-reject: END-OF-MESSAGE  5.7.1 Blocked by SpamAssassin

If you want the sender to see a different reject text, then add the -R (reject text) option like below.

OPTIONS="-u spamass-milter -i 127.0.0.1 -R SPAM_ARE_NOT_ALLOWED_HERE"

Save and close the file. Restart Postfix and Spamass Milter for the changes to take effect.

sudo systemctl restart postfix spamass-milter

Note: iRedMail users need to start the SpamAssassin service in order to use spamass-milter.

sudo systemctl start spamassassin
sudo systemctl enable spamassassin

Checking Email Header and Body with SpamAssassin

SpamAssassin ships with many spam detection rules in /usr/share/spamassassin/ directory. Allow me to explain some of the rules.

In the /usr/share/spamassassin/20_head_tests.cf file, you can find the following two lines.

header MISSING_HEADERS       eval:check_for_missing_to_header()
describe MISSING_HEADERS     Missing To: header

The first line tests if the To: header exists in an email message. The second line, which is optional, explains what the first line does. The uppercase letters is the name of this test.

The following 3 lines are for testing if there’s a Date: header in the email message.

header __HAS_DATE            exists:Date
meta MISSING_DATE            !__HAS_DATE
describe MISSING_DATE        Missing Date: header

And these 3 lines are for testing if there’s a From: header in the email message.

header __HAS_FROM            exists:From
meta MISSING_FROM            !__HAS_FROM
describe MISSING_FROM        Missing From: header

You might want to use the Cron job shipped with SpamAssassin to automatically update SpamAssassin’s rules on a daily basis. If so, open the /etc/default/spamassassin file and change CRON=0 to CRON=1.

Set Custom Score for Existing Rules

In the 50_scores.cf and 72_scores.cf file, you can see the default scores for various tests. If you think the default score is too low or too high for a certain test, you can set custom score in /etc/spamassassin/local.cf file.

sudo nano /etc/spamassassin/local.cf

For example, RFC 5322 requires that every email message must have From: and Date: header fields, so I can set a very high score if either of them is missing in an email message by appending the following two lines in local.cf file.

score MISSING_FROM   5.0
score MISSING_DATE   5.0

Although the To: header field is not mandatory in RFC 5322, I prefer to set a high score if it’s missing in an email message because I have never seen a legitimate email missing this header field.

score MISSING_HEADERS 3.0

Some spammers uses two email addresses in the From: header field like below.

From: "[email protected]" <[email protected]>

I think the default score for this kind of email is low, I prefer to set it to 3.0.

score PDS_FROM_2_EMAILS 3.0

There are spammers who send empty message with no subject and no textual parts in the body. I set the score for this kind of email to 5.0, so it will be placed to spam folder. Why read it if it’s empty?

score EMPTY_MESSAGE 5.0

And other spammers often ask you to send a read receipt, I set the score to 2.0 for this kind of email.

score FREEMAIL_DISPTO 2.0

There are some spammers use different domain names in the From: and Reply-To: header, I give them a 3.5 score.

score FREEMAIL_FORGED_REPLYTO 3.5

I also have seen some spammers using non-existent domain name in the From: header field. I set a 5.0 score for this type of email.

score DKIM_ADSP_NXDOMAIN 5.0

Last but not least, many spammers spoof the gmail.com domain in the From: header field. I set a 2.5 score this kind of email.

score FORGED_GMAIL_RCVD 2.5

Adding Your Own Rules

You can add custom SpamAssassin rules in /etc/spamassassin/local.cf file.

sudo nano /etc/spamassassin/local.cf

Header Rules

For example, some spammers use the same email address in the From: and To: header, you can add the following lines at the end of the file to add scores to such emails.

header   FROM_SAME_AS_TO   ALL=~/\nFrom: ([^\n]+)\nTo: \1/sm
describe FROM_SAME_AS_TO   From address is the same as To address.
score    FROM_SAME_AS_TO   2.0

Some spammers use an empty address for the Envelope From address (aka the Return Path header). Although this is legitimate for sending bounce messages, I prefer to give this kind of email a score.

header    EMPTY_RETURN_PATH    ALL =~ /<>/i
describe  EMPTY_RETURN_PATH    empty address in the Return Path header.
score     EMPTY_RETURN_PATH    3.0

If you have configured OpenDMARC on your mail server, you can now add the following lines to add scores to emails that fail DMARC check.

header    CUSTOM_DMARC_FAIL   Authentication-Results =~ /dmarc=fail/
describe  CUSTOM_DMARC_FAIL   This email failed DMARC check
score     CUSTOM_DMARC_FAIL   3.0

The above code tells SpamAssassin to check if the Authentication-Results header contains the string “dmarc=fail”. If found, increase the score by 3.0.

Body Rules

You can tell SpamAssassin to increase the score of an email if a certain phrase is found in the body. For example, many spammers use the recipient’s email address in the first body line like below.

Hi [email protected]
Hello [email protected]
Dear [email protected]

I don’t want to talk with people who doesn’t bother writing my name in the first line of email. So I created a rule in SpamAssassin to filter this kind of email.

body      BE_POLITE       /(hi|hello|dear) xiao\@linuxbabe\.com/i
describe  BE_POLITE       This email doesn't use a proper name for the recipient
score     BE_POLITE       5.0

Regular expression in SpamAssassin is case-sensitive by default, you can add the i option at the end to make it case-insensitive.

Add Negative Scores

You can also add negative score to good emails, so there will be less false positives. For example, many of my blog readers ask me Linux questions and I don’t think spammers would include words like Debian, Ubuntu, Linux Mint in the email body, so I created the following rule.

body      GOOD_EMAIL    /(debian|ubuntu|linux mint|centos|red hat|RHEL|OpenSUSE|Fedora|Arch Linux|Raspberry Pi|Kali Linux)/i
describe  GOOD_EMAIL    I don't think spammer would include these words in the email body.
score     GOOD_EMAIL    -4.0

If the email body contains a Linux distro’s name, then add a negative score (-4.0).

There are some common phrases that is included in legitimate bounce messages, so I can add negatives scores to these email messages.

body      BOUNCE_MSG    /(Undelivered Mail Returned to Sender|Undeliverable|Auto-Reply|Automatic reply)/i
describe  BOUNCE_MSG    Undelivered mail notifications or auto-reply messages
score     BOUNCE_MSG    -1.5

Note that body rules also include the Subject as the first line of the body content.

Meta Rules

In addition to header and body rules, there’s also meta rules. Meta rules are combinations of other rules. You can create a meta rule that fires off when two or more other rules are true. For example, I occasionally receive emails saying that the sender wants to apply for a job and a resume is attached. I have never said on my website that I need to hire people. The attachment is used to spread virus. I created the following meta rule to filter this kind of email.

body      __RESUME        /(C.V|Resume)/i
meta      RESUME_VIRUS    (__RESUME && __MIME_BASE64)
describe  RESUME_VIRUS    The attachment contains virus.
score     RESUME_VIRUS    5.5

The first sub rule __RESUME tests if the email body contains the word C.V. or resume. The second sub rule __MIME_BASE64 is already defined in /usr/share/spamassassin/20_body_tests.cf file, as follows, so I don’t need to define it again in local.cf file. This rule tests if the email message includes a base64 attachment.

rawbody   __MIME_BASE64  eval:check_for_mime('mime_base64_count')
describe  __MIME_BASE64  Includes a base64 attachment

My meta rule RESUME_VIRUS will fire off when both of the sub rules are true, adding a 5.5 score to the email message. Note that sub rule often starts with double underscore, so it has no score in its own right.

Now you learned how to add score if a string is found. What if you want to add score when a string doesn’t exist in the email headers? Well, you can use the ! operator. For example, I have seen spammers using a single word in the From: address.  I added the following lines to score this kind of email.

header __AT_IN_FROM   From =~ /\@/
meta  NO_AT_IN_FROM   !__AT_IN_FROM
score NO_AT_IN_FROM   4.0

The first line checks if the @ sign exists in the From: header. The second line defines a meta rule, which fires off when !__AT_IN_FROM is true. !__AT_IN_FROM rule is the opposite of the first header rule, which means when there’s no @ sign in the From: address, the meta rule fires off.

You can also add the following lines to check if a dot exists in the From: address.

header __DOT_IN_FROM   From =~ /\./
meta   NO_DOT_IN_FROM  !__DOT_IN_FROM
score  NO_DOT_IN_FROM  4.0

Whitelist

You can use the whitelist_from parameter to add a particular email address or domain to your Spamassassin whitelist. For example, add the following two lines at the end of local.cf file.

whitelist_from [email protected]
whitelist_from *@canonical.com

A whitelisted sender has a -100 default score. They will still be tested by SpamAssassin rules, but it’s super hard for them to reach a 5.0 score.

Blacklist

To blacklist a sender, use the blacklist_from parameter, which has the same format as whitelist_from.

blacklist_from [email protected]
blacklist_from *@example.org

Checking Syntax and Restart

After saving the local.cf file. You should run the spamassassin command in lint mode to check if there’s any syntax errors.

sudo spamassassin --lint

Then restart SpamAssassin for the changes to take effect. (If you use Amavis with Spamassasin as in iRedMail, you just need to restart Amavis: sudo systemctl restart amavis)

sudo systemctl restart spamassassin

SpamAssassin’s Builtin Whitelist

It’s worth mentioning that SpamAssassin ships with its own whitelist. There are several files under /usr/share/spamassassin/ directory that includes 60_whitelist in the filename. These files contain SpamAssassin’s builtin whitelist. For example, the 60_whitelist_spf.cf file contains a list of addresses which send mail that is often tagged (incorrectly) as spam.

Move Spam into the Junk Folder

Note: iRedMail has this configured out-of-the-box.

I’m going to show you how to move spam to Junk folder with the Dovecot IMAP server and the sieve plugin. This method requires that inbound emails are delivered to the message store via the Dovecot “deliver” LDA (local delivery agent). If you can find the following text in /var/log/mail.log file, then this requirement is satisfied.

postfix/lmtp

or

delivered via dovecot service

Run the following command install dovecot-sieve from Ubuntu software repository.

sudo apt install dovecot-sieve

This package installs two configuration files under /etc/dovecot/conf.d/ directory: 90-sieve.conf and 90-sieve-extprograms.conf. Open the 15-lda.conf file.

sudo nano /etc/dovecot/conf.d/15-lda.conf

Add the sieve plugin to local delivery agent (LDA).

protocol lda {
    # Space separated list of plugins to load (default is global mail_plugins).
    mail_plugins = $mail_plugins sieve
}

Save and close the file. If you can find the 20-lmtp.conf file under /etc/dovecot/conf.d/ directory, then you should also enable the sieve plugin in that file like below.

protocol lmtp {
      mail_plugins = quota sieve
}

Edit the /etc/dovecot/conf.d/10-mail.conf file.

sudo nano /etc/dovecot/conf.d/10-mail.conf

Sieve scripts are stored under each user’s home directory. If you followed my PostfixAdmin tutorial and are using virtual mailbox domains, then you need to enable mail_home for the virtual users by adding the following line in the file, because virtual users don’t have home directories by default.

mail_home = /var/vmail/%d/%n

Save and close the file. Then open the 90-sieve.conf file.

sudo nano /etc/dovecot/conf.d/90-sieve.conf

Go to line 79 and add the following line, which tells Sieve to always execute the SpamToJunk.sieve script before any user-specific scripts.

sieve_before = /var/mail/SpamToJunk.sieve

Save and close the file. Then create the sieve script.

sudo nano /var/mail/SpamToJunk.sieve

Add the following lines, which tells Dovecot to move any email messages with the X-Spam-Flag: YES header into Junk folder.

require "fileinto";

if header :contains "X-Spam-Flag" "YES"
{
   fileinto "Junk";
   stop;
}

Save and close the file. We can compile this script, so it will run faster.

sudo sievec /var/mail/SpamToJunk.sieve

Now there is a binary file saved as /var/mail/SpamToJunk.svbin. Finally, restart dovecot for the changes to take effect.

sudo systemctl restart dovecot

Set Message Maximum Size

By default, SpamAssassin does not check messages with attachments larger than 500KB, as indicated by the following line in the /var/log/mail.log file.

spamc[18922]: skipped message, greater than max message size (512000 bytes)

The default max-size is set to 512000 (bytes). A high value could increase server load, but I think the default size is a little bit small. To increase the max-size, edit /etc/default/spamass-milter file and add the following lines at the end.

#Spamc options
OPTIONS="${OPTIONS} -- --max-size=5120000"

The empty -- option tells spamass-milter to pass all remaining options to spamc, which understands the --max-size option. I increased the size to 5000KB. Save and close the file. Then restart spamass-milter.

sudo systemctl restart spamass-milter

How to Configure Individual User Preferences

You may want to set custom rules for emails sent to a specific address on the mail server. I like this feature very much. I have a contact email address for this blog, which is only used for keeping contact with readers. I don’t use the contact email address elsewhere, so I can create special spam-filtering rules that apply only to this contact email address.

First, edit the SpamAssassin main configuration file.

sudo nano /etc/spamassassin/local.cf

Add the following line to allow user rules.

allow_user_rules 1

Save and close the file. Next, edit the SpamAssassin environment file.

sudo nano /etc/default/spamassassin

Find the following line.

OPTIONS="--create-prefs --max-children 5 --helper-home-dir"

We need to change it to

OPTIONS="--create-prefs --max-children 5 --helper-home-dir --nouser-config --virtual-config-dir=/var/vmail/%d/%l/spamassassin --username=vmail"

Where:

  • --nouser-config: disable per-user configuration file for local Unix users.
  • --virtual-config-dir: specify the per-user configuration directory for virtual users. The %d placeholder represents the domain part of email address and %l represents the local part of email address.
  • --username: run spamd as the vmail user.

Save and close the file. Then restart SpamAssassin.

sudo systemctl restart spamassassin

By default, spamass-milter will send only the local part of email address to SpamAssassin. We need to make it send the full email address. Edit the spamass-milter configuration file.

sudo nano /etc/default/spamass-milter

Find the following line.

OPTIONS="-u spamass-milter -i 127.0.0.1 -R SPAM_ARE_NOT_ALLOWED_HERE"

Add the following option to this line.

-e yourdomain.com

Like this:

OPTIONS="-e yourdomain.com -u spamass-milter -i 127.0.0.1 -R SPAM_ARE_NOT_ALLOWED_HERE"

The -e option will make spamass-milter pass the full email address to SpamAssassin. Replace yourdomain.com with your real domain name. Save and close the file. Then restart spamass-milter.

sudo systemctl restart spamass-milter

Now send an email from Gmail, Hotmail, etc. to your domain email address. You will find the spamassassin directory is automatically created under /var/vmail/yourdomain.com/username/ directory.

cd /var/vmail/yourdomain.com/username/spamassassin/

You can use a command-line text editor to create the per-user preference file here. This file must be named as user_prefs.

sudo nano user_prefs

You can add custom rules in this file just as you would do in the /etc/spamassassin/local.cf file.

For instance, I found many spammers end their email body with an unsubscribe link to let you remove future contact. I didn’t subscribe to their spam and I don’t think the unsubscribe link will remove my email address from their contact database. So I use SpamAssassin to score this kind of email.  The following rule adds 3.0 score to emails containing the word “unsubscribe” or its variations in the body. (I don’t use the contact email address of this blog to subscribe to anything online.)

body      SUBSCRIPTION_SPAM   /(unsubscribe|u n s u b s c r i b e|Un-subscribe)/i
describe  SUBSCRIPTION_SPAM   I didn't subscribe to your spam.
score     SUBSCRIPTION_SPAM   3.0

Sometimes the email body doesn’t contain the word “unsubscribe”, but the there’s a List-Unsubscribe header, which means the spammer added my contact email address to their mailing list without my consent. I can score this type of email too, with the following rule.

header    LIST_UNSUBSCRIBE   ALL =~ /List-Unsubscribe/i
describe  LIST_UNSUBSCRIBE   I didn't join your mailing list.
score     LIST_UNSUBSCRIBE   2.0

I created a Mailjet account with a different email address. Some spammers assume that I used my contact email address to create Mailjet account, so they try to impersonate Mailjet customer service to lure me into typing my password at a fake Mailjet login page. I can score this kind of email like below.

header    MAILJET_IMPOSTER   From =~ /mailjet/i
describe  MAILJET_IMPOSTER   I don't have a mailjet account for this email address.
score     MAILJET_IMPOSTER   2.5

The above lines check if the From: header contains the word mailjet. If found, give it a 2.5 score.

I occasionally receive emails from Chinese spammers whose From: domain name has no vowel letters (a, e, i, o, u). The spammer used the cdjcbzclyxgs.xyz domain name. It is nearly impossible for a normal person/entity to use domain names without vowel letters, considering that many top level domains have already included vowel letters (.com, .net, .org, .co, .io, .shop, .dev, etc), so I give this kind of email a very high score like below. The default score is 0.5.

score FROM_DOMAIN_NOVOWEL 4.0

Some spam emails use many images in the body but contains very little text. The default score for this kind of email is 1.9, but I prefer to set a high score for my contact email address.

score HTML_IMAGE_RATIO_02 4.0

I also received a spam email with my email address in the subject, so I can add a high score to it.

header    SUBJECT_SPAM   Subject =~ /xiao\@linuxbabe.com/i
describe  SUBJECT_SPAM   Subject contains my email address.
score     SUBJECT_SPAM   4.0

Some spammers use BCC (Blind Carbon Copy) to hide other recipients. I don’t want to receive such email. So I made the following rule. If my domain name is not in the To: header, add 3.0 to the email.

header __DOMAIN_IN_TO     To =~ /linuxbabe.com/
meta   DOMAIN_NOT_IN_TO   !__DOMAIN_IN_TO
score  DOMAIN_NOT_IN_TO   3.0

After adding custom rules, close the file and run the following command to check syntax. Silent output means there’s no syntax error.

sudo spamassassin --lint

Finally, restart SpamAssassin for the changes to take effect.

sudo systemctl restart spamassassin

Now you can test the user preferences by sending test emails from other email services to your own domain email address.

Reject or Bounce

If a receiving SMTP server determines during the SMTP conversation that it will not accept the message, it rejects the message. Sometimes the SMTP server accepts a message and later discovers that it cannot be delivered, perhaps the intended recipient doesn’t exist or there is a problem in the final delivery. In this case, the SMTP server that has accepted the message bounces it back to the original sender by sending an error report, usually including the reason the original message could not be delivered.

You should not bounce spam, because the email address in the Return-path: header or From: header probably doesn’t exist or is an innocent person’s email address, so the bounce message will probably go to an innocent person’s email address, creating the backscatter problem. Instead of bouncing the spam, you should reject spam during the SMTP dialog, before the email is accepted. This article didn’t show you bouncing a spam message. You should remember this rule in case you are going to create spam-filter rules by yourself. If in doubt, test your spam-filtering rules to see if it’s going to create bounce messages.

URIBL_BLOCKED

By default, SpamAssassin enables URIBL rule, which checks if an email message contains links that are identified as spam by URIBL. This is a very effective anti-spam measurement. However, you might be blocked from querying URIBL. Check the raw email headers of an inbound email message, find the X-Spam-Status header.

X-Spam-Status: No, score=-92.2 required=5.0 tests=DATING_SPAM,DKIM_SIGNED,
	DKIM_VALID,HTML_FONT_LOW_CONTRAST,HTML_MESSAGE,SPF_PASS,
	SUBSCRIPTION_SPAM,UNPARSEABLE_RELAY,URIBL_BLOCKED,USER_IN_WHITELIST
	autolearn=no autolearn_force=no version=3.4.2

If you can find URIBL_BLOCKED in this header, that means you are blocked from querying URIBL. Most of the time it’s because you are not using your own local DNS resolver. You can run the following command on your mail server to test which DNS server you are using to query URIBL.

host -tTXT 2.0.0.127.multi.uribl.com

Sample output:

2.0.0.127.multi.uribl.com descriptive text "127.0.0.1 -> Query Refused. See http://uribl.com/refused.shtml for more information [Your DNS IP: xx.xx.xx.xx]"

To fix this error, you need to run your own local DNS resolver on your mail server.

Once your local DNS resolver is up and running, test URIBL again.

host -tTXT 2.0.0.127.multi.uribl.com

If you see the following output, it means you are now allowed to query URIBL.

2.0.0.127.multi.uribl.com descriptive text "permanent testpoint"

From here on out, inbound email messages won’t have the URIBL_BLOCKED tag in the X-Spam-Status header.

Prevent Outgoing Spam

Postfix header and body checks can also be used to prevent outgoing spam, but we need to use smtp_header_checks, instead of header_checks, and smtp_body_checks instead of body_checks, because smtp_header_checks and smtp_body_checks are only applied when Postfix is acting as an SMTP client, so it won’t affect incoming emails.

For example, if you don’t want your email server to send emails to a particular email address, you can create the /etc/postfix/smtp_header_checks file and add the following line to it.

/^To:.*fake.*/       DISCARD

The above line tests if the To: header contains the fake word. If found, discard the email. I’m just showing a simple example here. This can be useful when you have a contact form or sign-up form on your website and some visitors use fake email addresses on the form to generate spam on your email server.

Save and close the file. Then edit the Postfix main configuration file.

sudo nano /etc/postfix/main.cf

Add the following line at the end of the file.

smtp_header_checks = pcre:/etc/postfix/smtp_header_checks

Save and close the file. Then run the following command to rebuild the hash table.

sudo postmap /etc/postfix/smtp_header_checks

Reload Postfix for the change to take effect.

sudo systemctl reload postfix

Deleting Email Headers

You can also use smtp_header_checks to delete email headers that could show sensitive information. For example, you might not want the recipient to know that you are using SpamAssassin on your mail server, then you can edit the /etc/postfix/smtp_header_checks file and add the following lines in the file. This tells Postfix to delete the X-Spam-Status and X-Spam-Checker-Version header from the email message when sending emails.

/^X-Spam-Status:/             IGNORE
/^X-Spam-Checker-Version:/    IGNORE

Save and close the file. Then run the following command to rebuild the hash table.

sudo postmap /etc/postfix/smtp_header_checks

Reload Postfix for the change to take effect.

sudo systemctl reload postfix

Other Tidbits

The spamassassin package on Debian/Ubuntu ships with a Cron job (/etc/cron.daily/spamassassin) to automatically update rulesets daily with the sa-update command.

SpamAssassin 4.0 includes a HashBL plugin, which can check if a Bitcoin address in the email body has been used by scammers. And there’s also a new plugin called “Ole Macro” that can check if an email contains an Office attachment with a macro. This plugin would try to detect if the attched macro is malicious or not.

Wrapping Up

I hope this tutorial helped you use Postifix and SpamAssassin to filter spam. As always, if you found this post useful, then subscribe to our free newsletter to get more tips and tricks. Take care 🙂

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43 Responses to “Block Email Spam with Postfix and SpamAssassin Content Filter

  • Marcio Merlone
    1 year ago

    Very good guide, although it is not wise to block/reject empty addresses, like From: . There is legitimate mail with that.

    • Yes, legitimate bounce messages and auto-reply messages can use empty MAIL FROM address (Return-Path: <>), but they still use a valid email address in the From: header.

      MAIL FROM is different than Header From. This article only shows rejecting empty Header From.

  • Nina Vise
    1 year ago

    Hey Xiao Guo An,

    very nice guide I just used the Part: Set Reject Score for my Plesk installation of dovecot and postfix, and it is working. Is there a way to change the Message “Blocked by SpamAssassin” to “Blocked by provider in case you are a human contact: xyz” ?

    Thank you very much!

    • Edit the /etc/default/spamass-milter file, add the -R (reject text) option like below.

      OPTIONS="-u spamass-milter -i 127.0.0.1 -R your_custom_reject_text"

      Then restart spamass-milter.

      sudo systemctl restart spamass-milter

      Note that you can’t add space character in your reject text, or only the first word will be seen by the sender. I haven’t found a way to escape the space character yet, so I use underscore in place of space character.

  • t0m_Cr00z
    11 months ago

    Hi Bro Xiao Guo An,

    Is this ok to apply this with my current setup of iredmail? Which i also follow in one of your articles.

  • I was getting a lot of spam recently mostly from same template.
    I added the body_checks regex. let’s see if i get same spam again.
    thanks for the tutorial though.. 🙂

  • Hi Bro Xiao Guo An!
    I have this problem. When I try to execute query: doveadm expunge -A mailbox Junk all, I get this error – Error: User listing returned failure

    doveadm: Error: Failed to iterate through some users.

    In auth-sql.conf.ext

    passdb {
      driver = sql
      args = /etc/dovecot/dovecot-sql.conf.ext
    }
    userdb {
      driver = static
      args = uid=vmail gid=vmail home=/var/mail/vhosts/%d/%n
    }

    and i have

    iterate_query = SELECT email AS user FROM virtual_users; 

    in /etc/dovecot/dovecot-sql.conf.ext But requests are executed with an error

    dovecot - auth: Error: Trying to iterate users, but userdbs don't support it
    • If you are using virtual mailbox with MySQL/MariaDB, then the driver should be set to sql.

      userdb {
        driver = sql
        args = /etc/dovecot/dovecot-sql.conf.ext
      }
      
  • Guy Merritt
    5 months ago

    I was very excited to find a word, or string-based, content filter for Postfix. After using Qmail for years, I had to make a switch on my system as Qmail is not maintained and won’t even compile on modern Linux systems, now. Spamassassin works fine, but when I add the header_checks and body_checks lines to main.cf, my Postfix system goes into fits – it seem that some sort of loop is being created, plus, I get an error email, part of which says:

     Out: 250 2.1.5 Ok
     In:  DATA
     Out: 354 End data with .
     Out: 451 4.3.0 Error: queue file write error
     In:  QUIT
     Out: 221 2.0.0 Bye

    I’m an old guy – 68 in 10 days – and I’m not smart enough to have any clue as to why this is happening. If I send one email with these features enabled, I get an error email – like the one I’ve posted – about every 3 minutes, in my inbox. If I remove those lines and reload Postfix, everything works fine. Any idea why this might be happening? I did add a line to increase the size limit of the queue to something huge, but that didn’t change this weird behavior I’m seeing when I attempt to use these features.

    • Guy Merritt
      5 months ago

      Okay……found ONE page on the web that suggested a different syntax and now everything is working perfectly – I had to use the following syntax for the entries on the bottom of the /etc/postfix/main.cf file:

      body_checks = regexp:/etc/postfix/body_checks

      I’m a dummy – have no idea what the “regexp” means, nor, does. But my mail server, now, either dumps (discards) filtered email in both the body and head….or it will reject the offending emails and notify the sender. No errors, no issues when I use this syntax.

    • Perhaps it’s because your system doesn’t have PCRE map support for Postfix. on Debian and Ubuntu, you can run the following command to install it.

      sudo apt install postfix-pcre
  • Guy Merritt
    5 months ago

    I had followed your “recipe” and, indeed, had installed postfix-pcre. I also ran the command (don’t recall the command) to see if pcre was available to Postfix – the results listed Postfix as having pcre as an available component. So, heck – I don’t know why it didn’t work. But your article is excellent, anyway. Thanks for putting such good info together for the “slow” people, like me! Anyway, in the event that things don’t work in the way you’ve suggested the method I found is a completely workable alternative. Thanks again for your good work.

  • Guy Merritt
    5 months ago

    How can a person support your work…?? Is there a place to donate?

  • Guy Merritt
    5 months ago

    Well…..you won’t be able to retire on a whopping 20 bucks, but – hey – it’s the end of the month!!!!!

  • Thank your for all the great tutorials… I’ve used several of them. In my old mail server I used regexp for my header_checks so I love the speed increase of using pcre. My question has to do with comments within the header_checks file. when I was using regexp I would add something like: “# rule to stop annoying diet pill emails”.

    # /^HEADER: .*content_to_act_on/ ACTION
    # /^Subject .*Free Money/ REJECT Spam Header Rule #42
    /^Subject: .*Make Money Fast*/ REJECT
    /^Subject: .*Dr\. Oz*/ REJECT
    

    Will it cause a problem if I include comments with # in front of them?

    • A line starting with # is a comment and the remaining regex in the line is ignored.

      It’s better to place the comment on its own line.

      # Spam Header Rule #42
      /^Subject .*Free Money/    REJECT
      
      • Thank you for your help. I really appreciate your explanations within your tutorials instead of just telling us what setting to use. Its nice to know what each setting does so we can make intelligent decisions about how the pieces go together.

  • – Is there any advantage to upgrade those components – ubuntu, dovecot, postfix, postfiadmin, nginx, openssl, to the latest versions?
    – If yes how can we perform those tasks?
    Greetings

    • If you don’t know any advantage of using latest version, then you should use the default version in Ubuntu repository. I only use the latest version when it has a new feature that I really need.

      • But If We leave this installation as is, very soon it will become older and useless. Any installation needs a plan to have the most reliable versions. That’s what I was talking about and asking for, thank you.

    • Ubuntu LTS is supported for 5 years. It is reliable and not going to be useless anytime soon. When the next Ubuntu LTS comes out, simply follow a tutorial to upgrade to it.

      • What about the other components – dovecot, postfix, nginx, MariaDB, PHP, … -? It is advised to upgrade to the latest versions OR the actual versions are good for the next 5 years as ubuntu LTS?

    • Both the system and any software in the repository are supported for 5 years.

      You don’t need to upgrade every software to the latest version on a production server.

      Actually it’s a bad practice to use the latest version for every software on a production server. Latest version is usually less reliable than previous versions.

  • Hi, have a doubt regarding this rule
    /free mortgage quote/ DISCARD

    Can I use one like this
    /free mortage/quote/ DISCARD

    Regards

  • First of all thank you for these detailed tutorials. I want to point out that this is the first time I am making a mail server, I do not have much experience.
    I have all your tutorials on how to activate port 465 etc. Modoboa worked on sending and receiving messages, configured on the Iphone mail client.
    After that I did the test at: https://www.mail-tester.com and the score was 6/10
    Here are the test issues:
    most important point 1.
    1.

    SpamAssassin thinks you can improve
    -4
    The famous spam filter SpamAssassin. Score: -4.
    A score below -5 is considered spam<
    
    0.1		DKIM_SIGNED		Message has a DKIM or DK signature, not necessarily valid.This rule is automatically applied if your email contains a DKIM signature but other positive rules will also be added if your DKIM signature is valid. See immediately below.
    DKIM_VALID		Message has at least one valid DKIM or DK signatureGreat! Your signature is valid
    0.1		DKIM_VALID_AU		Message has a valid DKIM or DK signature from author's domain Great! Your signature is valid and it's coming from your domain name
    0.1		DKIM_VALID_EF		Message has a valid DKIM or DK signature from envelope-from domain 
    -1.985		PYZOR_CHECK		Similar message reported on Pyzor (http://pyzor.org) https://pyzor.readthedocs.io/en/latest/
    Please test a real content, test Newsletters will always be flagged by Pyzor Adjust your message or request whitelisting (http://public.pyzor.org/whitelist/)
    -1.274		RDNS_NONE		Delivered to internal network by a host with no rDNS This may indicate you do not have a rDNS configured for your hostname or the rDNS does not match your sending IP
    -0.896		SPF_HELO_SOFTFAIL		SPF: HELO does not match SPF record (softfail) softfail
    0.001		SPF_PASS		SPF: sender matches SPF record Great! Your SPF is valid

    2.
    2.1

    Your message could be improved
    There is no html version of your message.
    
    You're not fully authenticated 

    We check if the server you are sending from is authenticated

    2.2

     Your reverse DNS does not match with your sending domain.
    Reverse DNS lookup or reverse DNS resolution (rDNS) is the determination of a domain name that is associated with a given IP address. Some companies such as AOL will reject any message sent from a server without rDNS, so you must ensure that you have one.
    You cannot associate more than one domain name with a single IP address.

    3.

    Your message does not contain a List-Unsubscribe header
    The List-Unsubscribe header is required if you send mass emails, it enables the user to easily unsubscribe from your mailing list.
    Your message does not contain a List-Unsubscribe header

    I did the changes according to your tutorial. From this moment, I manage to send messages but I do not receive any messages sent to the domain installed on Modoboa.
    I try again the test and …
    Consider yourself lucky if your email goes to an inbox 4.8/10

    Can you help me??

  • michael rowe
    3 months ago

    Great Article. Incorporated a lot of it. It put a lot of things all in one place. Thanks

  • Raffael Carvalho
    3 months ago

    Hello! Due to an AWS SES restriction I need to create a routine to block the sending of email to email addresses that have marked the receipt of email from my server as spam. The ideal would be to block only the sending by the recipient of the email returned as spam.
    This is my first email server, and I practically did everything following your tutorials (postfix / dovecot / roundcube). Can you give me a tip on how to create this block / rule?

  • Erik van de Stolpe
    3 months ago

    Hi Xiao, First of all: a big THANK YOU for your mailserver series (from scratch, Ubuntu). In this last episode of the series, you tell us in the “Integrate SpamAssassin with Postfix SMTP Server as a Milter” parapraph that we should now have the following line for the smtpd_milters if the series was followed:

    smtpd_milters = local:opendkim/opendkim.sock,local:opendmarc/opendmarc.sock,local:spamass/spamass.sock

    I followed the series, but I don’t recall having seen any addition of local:opendkim/opendkim.sock or local:opendmarc/opendmarc.sock. Mine was empty. Upon adding the suggested line, sending mail stopped working with the error:

    sieve script file path ~/.dovecot.sieve is relative to home directory, but home directory is not available.

    So I quickly reverted to my snapshot to get things working again.

    Did I miss something here?

    • If you followed part 3, you should have the opendkim milter.

      And this OpenDMARC tutorial shows you how to integrate OpenDKIM with Postfix via the milter interface.

      You can set a home directory for mail users in /etc/dovecot/conf.d/10-mail.conf file. Add the following line in this file.

      mail_home = /var/vmail/%d/%n

      Then restart dovecot.

      sudo systemctl restart dovecot

      PS: I didn’t set the home direcory for mail users on my mail server, and my sieve scripts are working fine.

      • Erik van de Stolpe
        3 months ago

        Apart from having a flu, I must have been drunk. I’ll go over the whole series once more to find if I missed more.

  • Hi,
    It was all working well until I added the spam assassin and connected plugins in. Now my emails are not getting through. My gmail acct. was getting through before.
    These are the errors in my mail.log file.
    I see from the last comment the person before left which sounded similar. Although I have followed yours from start to finish.
    Here is my amended log file:

    Mar 31 18:05:20 mail dovecot: auth: Debug: userdb out: USER#0111#011[MYEMAIL]#011maildir=[MYDOMAIN]/[USR]/#011uid=2000#011gid=2000
    Mar 31 18:05:20 mail dovecot: lmtp(MYEMAIL): Error: User [EMAIL] doesn't have home dir set, disabling duplicate database
    Mar 31 18:05:20 mail dovecot: lmtp(MYEMAIL): Error: sieve: file storage: Sieve storage path `~/sieve' is relative to home directory, but home directory is not available.
    Mar 31 18:05:20 mail dovecot: lmtp(MYEMAIL): Error: sieve: Failed to access user's personal storage (temporary failure)
    Mar 31 18:05:20 mail dovecot: lmtp(12805): Disconnect from local: Client has quit the connection (state=READY)
    Mar 31 18:05:20 mail postfix/lmtp[12803]: 962C5817A7: to=, relay=[SERVER][private/dovecot-lmtp], delay=12965, delays=12965/0.04/0.03/0.09, dsn=4.2.0, status=deferred (host [SERVER][private/dovecot-lmtp] said: 451 4.2.0  Temporarily unable to access necessary Sieve scripts (in reply to end of DATA command))

    Any help would be gratefully received

    • Hello Xiao Guoan – I really appreciate that you look through and give real answers to your readers. So thanks…
      so hopefully on my other recent comments you can see I really try to resolve my issues to stop you trying to answer my solved problems. I think I have ‘temporarily’ resolved the issue…

      At this point the I have been unsuccessfully sending through the day (messages I thought lost) are pinging through. removing sieve from this command:
      The only change I can think I made that could have solved it is either removing sieve from this command:

      protocol lmtp {
            mail_plugins = quota sieve
      }

      or removing a mail2.mydomain.com MX 5 mail.mydomain.com line from my DNS (this was to allow the fail2ban protocols in your ‘7 effective tips…’ page

    • Your mail server’s problem is that there’s no home directory for virtual users. Sieve scripts are stored under each user’s home directory. If you followed my PostfixAdmin tutorial and are using virtual mailbox domains, then you need to enable mail_home for virtual users by adding the following line in the file, because virtual users don’t have home directories by default.

      mail_home = /var/vmail/%d/%n

      Save and close the file. And restart Dovecot.

      sudo systemctl restart dovecot

      Now you can re-enable sieve filter.

      • Hi I added this line (as per your other article), but for me, it doesn’t work. I changed it to:

        mail_home = maildir:/var/vmail/%d/%n

        This gets rid of most errors except the sieve command.

      • Sorted – Thanks…

  • Firstly, thanks for an incredible resource this has been so helpful getting things set up.

    I seem to be having problems with spamassassin though. I’m running Ubuntu 18.04 and then followed your instructions to install iRedMail and then all the other elements.

    When I receive an email I get the following error:

    spamd[14648]: plugin: eval failed: bayes: (in learn) locker: safe_lock: cannot create lockfile /var/vmail/[MyDomain]/[Username]/spamassassin/bayes.mutex: No such file or directory

    The /var/vmail directory structure I’ve got doesn’t match the one in your example, mine is /var/vmail/vmail1/[MyDomain]/[X]/[Y]/[Z]/[Username]-[date time]/ where X, Y & Z are the first three characters of the Username. Below the Username folder are Maildir and sieve directories.

    If you can sort this out for me there’ll efinitely be a couple of cappuccinos in it.

    Many thanks

  • Ron Jones
    4 weeks ago

    Excellent work!

    Does the latest iteration of spamassassin automatically learn the spam tokens? Or do we need to set up a cron job for sa-learn?

    Like this example from a previous server of mine:

    11 03 7,14,21,28 * * /usr/bin/sa-learn --spam "/var/vmail///.Junk/cur/"

    Followed by another that empties the spam folder after learning this week’s spam:

    31 03 7,14,21,28 * * cd /var/vmail///.Junk/cur/ && /bin/rm -f *

    Thanks again,
    Ron

    • In my entry above, I used carat marks to enclose domain & username, and I don’t think the comment system liked that.

      Both examples should read as follows.

      "/var/vmail/mydomain.com/username/.Junk/cur/"
      • I am also looking forward to the answer. @Ron, have you tried or found anything out in the last weeks?

        Cheers!

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