Build Your Email Server on Ubuntu Part 2: Setting up DKIM and SPF
In the 1st part of our build your own email server on ubuntu tutorial series, we have a basic Postfix email server up and running and can send plain text email, read incoming emails using the command line.
I tested that tutorial again on my Ubuntu 16.04 server and found that although I set the correct MX, A and PTR record, my emails were flagged as spam by Gmail and Outlook mail. So in part 2, we are going to look at how to ensure email delivery to recipient’s inbox by setting up SPF and DKIM records.
What are SPF and DKIM Records?
SPF and DKIM are two types of TXT records in DNS that can help prevent email spoofing and ensure legitimate emails are delivered into the recipient’s inbox instead of spam folder. If your domain is abused by email spoofing, then your emails are likely to landed in recipient’s spam folder if they didn’t add you in address book.
SPF (Sender Policy Framework) record specifies which hosts or IP address are allowed to send emails on behalf of a domain. You should allow only your own email server or your ISP’s server to send emails for your domain.
DKIM (DomainKeys Identified Mail) use a private key to add a signature to emails sent from your domain. Receiving SMTP server verify the signature using the pubic key which is published in your DNS manager.
Create an SPF Record in DNS
In your DNS management interface, create a new TXT record like below.
TXT @ v=spf1 mx ~all
- TXT indicates this is a TXT record.
- Enter @ in the name field.
- v=spf1 indicates this is a SPF record and the SPF record version is SPF1.
- mx means all hosts listed in the MX records are allowed to send emails for your domain and all other hosts are disallowed.
- ~all indicates that emails from your domain should only come from hosts specified in the SPF record. Emails that are from other hosts will be flagged as forged.
Note that some DNS managers require you to wrap the SPF record with quotes like below.
TXT @ "v=spf1 mx ~all"
To check if your SPF record is propagated to the public Internet, you can use the dig utility on your Linux machine like below:
dig your-domain.com txt
txt option tells
dig that we only want to query TXT records.
You can also use online SPF validator such as spf.myisp.ch to see which hosts are allowed to send emails for your domain and debug your SPF record if any error occurs. The dmarcian SPF surveyor can help test your SPF record syntax.
Configuring SPF Policy Agent
We also need to tell our Postfix SMTP server to check for SPF record of incoming emails. This doesn’t help ensure outgoing email delivery but help with detecting forged incoming emails.
First install required packages:
sudo apt install postfix-policyd-spf-python
Then edit the Postfix master process configuration file.
sudo nano /etc/postfix/master.cf
Add the following lines at the end of the file.
policyd-spf unix - n n - 0 spawn user=policyd-spf argv=/usr/bin/policyd-spf
Save and close the file.
Next, edit Postfix main configuration file.
sudo nano /etc/postfix/main.cf
Append the following lines at the end fo the file.
policyd-spf_time_limit = 3600 smtpd_recipient_restrictions = reject_unauth_destination, check_policy_service unix:private/policyd-spf
The first line specifies the Postfix policy agent timeout setting. The following lines will add restriction on incoming emails by rejecting unauthorized email and checking SPF record.
Save and close the file. Then restart Postfix.
sudo service postfix restart
sudo systemctl restart postfix
Next time, when you receive an email from a domain that has an SPF record, you can see the SPF check results in the raw email header. The following header indicates a successful check against SPF.
Received-SPF: Pass (sender SPF authorized).
You can also send an email from your domain to Gmail or Microsoft Mail to see if SPF check can be passed. It’s worth noting that setting an SPF record alone won’t ensure your email land in recipient’s inbox.
Setting up DKIM
First install OpenDKIM which is an open source implementation of the DKIM sender authentication system.
sudo apt install opendkim opendkim-tools
postfix user to
sudo gpasswd -a postfix opendkim
Edit OpenDKIM main configuration file.
sudo nano /etc/opendkim.conf
Uncomment the following lines. Replace
Canonicalization simple Mode sv SubDomains no
Then add the following lines below
#ADSPAction continue line. If your file doesn’t have
#ADSPAction continue line, then just add them below
AutoRestart yes AutoRestartRate 10/1M Background yes DNSTimeout 5 SignatureAlgorithm rsa-sha256
Add the following lines at the end of this the file.
#OpenDKIM user # Remember to add user postfix to group opendkim UserID opendkim # Map domains in From addresses to keys used to sign messages KeyTable /etc/opendkim/key.table SigningTable refile:/etc/opendkim/signing.table # Hosts to ignore when verifying signatures ExternalIgnoreList /etc/opendkim/trusted.hosts InternalHosts /etc/opendkim/trusted.hosts
The final configuration file is as follows:
# This is a basic configuration that can easily be adapted to suit a standard # installation. For more advanced options, see opendkim.conf(5) and/or # /usr/share/doc/opendkim/examples/opendkim.conf.sample. # Log to syslog Syslog yes # Required to use local socket with MTAs that access the socket as a non- # privileged user (e.g. Postfix) UMask 002 # Sign for example.com with key in /etc/mail/dkim.key using # selector '2007' (e.g. 2007._domainkey.example.com) #Domain example.com #KeyFile /etc/mail/dkim.key #Selector 2007 # Commonly-used options; the commented-out versions show the defaults. Canonicalization relaxed/simple Mode sv SubDomains no #ADSPAction continue AutoRestart yes AutoRestartRate 10/1M Background yes DNSTimeout 5 SignatureAlgorithm rsa-sha256 # Always oversign From (sign using actual From and a null From to prevent # malicious signatures header fields (From and/or others) between the signer # and the verifier. From is oversigned by default in the Debian pacakge # because it is often the identity key used by reputation systems and thus # somewhat security sensitive. OversignHeaders From # List domains to use for RFC 6541 DKIM Authorized Third-Party Signatures # (ATPS) (experimental) #ATPSDomains example.com #OpenDKIM user # Remember to add user postfix to group opendkim UserID opendkim # Map domains in From addresses to keys used to sign messages KeyTable /etc/opendkim/key.table SigningTable refile:/etc/opendkim/signing.table # Hosts to ignore when verifying signatures ExternalIgnoreList /etc/opendkim/trusted.hosts InternalHosts /etc/opendkim/trusted.hosts
Save and close the file.
Create Signing table, key table and trusted hosts file
Create a directory structure for OpenDKIM
sudo mkdir /etc/opendkim sudo mkdir /etc/opendkim/keys
Change owner from
opendkim and make sure only
opendkim user can read and write to the keys directory.
sudo chown -R opendkim:opendkim /etc/opendkim sudo chmod go-rw /etc/opendkim/keys
Create the signing table.
sudo nano /etc/opendkim/signing.table
Add this line to the file. Replace your-domain.com with your real domain.
Then create the key table.
sudo nano /etc/opendkim/key.table
Add the following line. Replace your-domain with your actual domain.
Save and close the file.
Configure Trusted Hosts
Create the file.
sudo nano /etc/opendkim/trusted.hosts
Add the following lines to the newly created file.
127.0.0.1 localhost *.your-domain.com
The above means that messages coming from the above IP addresses and domains will be trusted and signed.
Generate Private/Public Keypair
Since DKIM is used to sign outgoing messages and verify incoming messages, we need to generate a private key for signing and a public key for remote verifier. Public key will be published in DNS.
Create a separate folder for the domain.
sudo mkdir /etc/opendkim/keys/your-domain.com
Generate keys using
sudo opendkim-genkey -b 2048 -d your-domain.com -D /etc/opendkim/keys/your-domain.com -s default -v
The above command will create 2048 bits keys.
-d (domain) specifies the domain.
-D (directory) specifies the directory where the keys will be stored and we use
default as the
selector (-s), also known as the name. Once the command is executed, the private key will be
default.txt will be the TXT record that contains public key.
opendkim as the owner of the private key.
sudo chown opendkim:opendkim /etc/opendkim/keys/your-domain.com/default.private
Add Public Key in DNS Records
Display the public key
sudo cat /etc/opendkim/keys/your-domain.com/default.txt
The string after the
p parameter is the public key.
In you DNS manager, create a TXT record, enter
default._domainkey in the name field. Then copy everything in the parentheses and paste into the value field. Delete all double quotes and white spaces.
Test your configuration
Enter the following command on Ubuntu 16.04 server to test your key.
sudo opendkim-testkey -d your-domain.com -s default -vvv
If everything is OK, you will see
Connect Postfix to OpenDKIM
Create a directory to hold the OpenDKIM socket file and only allow opendkim user and postfix group to access it.
sudo mkdir /var/spool/postfix/opendkim sudo chown opendkim:postfix /var/spool/postfix/opendkim
Then edit the socket configuration file.
sudo nano /etc/default/opendkim
Uncomment the first SOCKET line and replace it with the following line.
Edit Postfix main configuration file.
sudo nano /etc/postfix/main.cf
Add the following lines after
# Milter configuration # OpenDKIM milter_default_action = accept milter_protocol = 2 smtpd_milters = local:/opendkim/opendkim.sock non_smtpd_milters = local:/opendkim/opendkim.sock
Save and close the file. Then restart
sudo service opendkim restart sudo service postfix restart
sudo systemctl restart opendkim sudo systemctl restart postfix
You can send a test email to
[email protected] and get a free email authentication report.
echo "test email" | sendmail [email protected]
Here’s the report I got from port25.com
========================================================== Summary of Results ========================================================== SPF check: pass DomainKeys check: neutral DKIM check: pass Sender-ID check: pass SpamAssassin check: ham
You can see that my email passed both SPF and DKIM check. DomainKeys is the forerunner to DKIM and it is obsolete now. You don’t need to worry about it. ham, which means the email is not spam, is a terminology used by Apache SpamAssassin.
You can also send a test email to your Gmail account to see if DKIM test is passed.
echo "test email" | sendmail email@example.com
If you click the
show original button in Gmail, you should see
In my test, the email landed in my Gmail inbox. However, it’s stilled labeled as spam in my outlook.com email although both SPF and DKIM are passed.
If your message is not signed and DKIM check failed, you may want to check postfix log (
/var/log/mail.log) to see what’s wrong in your configuration.