Use Postfix Transport Map & Relayhost Map For Flexible Email Delivery

In previous tutorials, we discussed how to quickly set up a full-featured mail server using iRedMail or Modoboa, and we also learned how to set up SMTP relay with Postfix SMTP server to bypass port 25 blocking or IP blacklists. However, you might not want to set up your entire email server to use a relay host. We can configure Postfix transport map and relay map so that some emails are delivered via relay host, other emails are sent directly to recipients.

There are many SMTP relay services. In this tutorial, I use Sendinblue which allows you to send 9000 emails per month for free. SMTP relay services maintain a good IP reputation. They stop the bad senders and grow the good ones, so using SMTP relay service will increase the chance of hitting the inbox.

How to Use Postfix Transport Map

The transport map defines mappings from recipient address to transport method. By default, the value of the transport_maps parameter in Postfix is not set, as can be checked with:

postconf transport_maps

Sample output:

transport_maps =

iRedMail and Modoboa use MySQL/MariaDB database to store transport maps. If you used iRedMail to set up your mail server, then the output should be like:

transport_maps = proxy:mysql:/etc/postfix/mysql/transport_maps_user.cf proxy:mysql:/etc/postfix/mysql/transport_maps_domain.cf

If you used Modoboa to set up your mail server, then the output should be like:

transport_maps = proxy:mysql:/etc/postfix/sql-transport.cf proxy:mysql:/etc/postfix/sql-spliteddomains-transport.cf

The transport_maps parameter points to one or more transport lookup tables. You can edit the Postfix main configuration file.

sudo nano /etc/postfix/main.cf

And set a value by adding the following line at the end of the file. The /etc/postfix/transport file will contain the lookup table.

transport_maps = hash:/etc/postfix/transport

If you use iRedMail, you can find the transport_maps parameter and set the value to

transport_maps =
    hash:/etc/postfix/transport
    proxy:mysql:/etc/postfix/mysql/transport_maps_user.cf
    proxy:mysql:/etc/postfix/mysql/transport_maps_domain.cf

If you use Modoboa, you can find the tranport_maps parameter and set the value to:

transport_maps =
        hash:/etc/postfix/transport
        proxy:mysql:/etc/postfix/sql-transport.cf
        proxy:mysql:/etc/postfix/sql-spliteddomains-transport.cf

A lookup table can be a file, or in the form of MySQL/MariaDB database tables. Lookup tables will be searched in the specified order until a match is found.

Save and close the Postfix main configuration file. Next, we need to create the lookup table file.

sudo nano /etc/postfix/transport

In this file, we can define mappings from recipient addresses to transport methods. For example, I found that many .pl (Poland) domains are using a particular blacklist that blocks my mail server’s IP address. I can add the following line in this file so that emails sent to .pl domains will be relayed through Sendinblue.

.pl                relay:[smtp-relay.sendinblue.com]:587

Some people find that it’s hard to get into the inbox of Microsoft mailboxes (hotmail.com, outlook.com, etc). It’s very likely that your email will be put in the spam folder. Well, you can try using Sendinblue to deliver emails to Microsoft mailbox users. Sendinblue even allows you to see if the recipient opened or clicked links in your email. So I put the following lines in the file.

/.*@hotmail.*/i             relay:[smtp-relay.sendinblue.com]:587
/.*@outlook.*/i             relay:[smtp-relay.sendinblue.com]:587
/.*@live.*/i                relay:[smtp-relay.sendinblue.com]:587
/.*@msn.*/i                 relay:[smtp-relay.sendinblue.com]:587

If you want to use relay host to deliver emails to a particular recipient, but send emails directly to all other recipients in the same domain, then you can add a line like below.

[email protected]           relay:[smtp-relay.sendinblue.com]:587

If a certain SMTP server doesn’t use the default SMTP port 25, but uses a different port such as 2525 to receive incoming emails, you can add the following line

example.com        smtp:[mail.example.com]:2525

It’s a good practice to add your own domain name in this file like below.

your-domain.com    local

This tells Postfix that emails sent to your own domain should be delivered locally. This is the default behavior for canonical domains. If your mail server has multiple virtual domains, you should add all of your virtual domains.

your-domain1.com         local
your-domain2.com         local

If you just put the following two lines in the file and don’t add other lines, this will make all emails, excluding emails sent to your own domain, delivered via the relay host. The asterisk (*) is a wild-card character that represents any email address.

your-domain.com       local
*                     relay:[smtp-relay.sendinblue.com]:587

We can also have the following configuration, which means that emails sent to your own domain are delivered locally. Email sent to gmail.com are delivered normally by performing MX lookup and all other emails are delivered via the relay host.

your-domain.com       local 
gmail.com             smtp
*                     relay:[smtp-relay.sendinblue.com]:587

Save and close the file. Then run the following command to build the index file.

sudo postmap /etc/postfix/transport

Restart Postfix for the changes to take effect.

sudo systemctl restart postfix

Sender Dependent Relay Maps

The transport map defines mappings from a recipient address to transport method. If you want to define mappings from sender address to relay hosts, use the sender_dependent_relay_maps parameter. By default, its value is empty, as can be seen with:

postconf sender_dependent_relayhost_maps

Output:

sender_dependent_relayhost_maps =

iRedMail uses MySQL/MariaDB database to store sender-dependent relayhost maps. If you used iRedMail to set up your mail server, then the output should be like:

sender_dependent_relayhost_maps = proxy:mysql:/etc/postfix/mysql/sender_dependent_relayhost_maps.cf

The sender_dependent_relayhost_maps parameter points to one or more lookup tables. You can edit the Postfix main configuration file.

sudo nano /etc/postfix/main.cf

And set a value by adding the following line at the end of the file. The /etc/postfix/relay_by_sender file will contain the lookup table.

sender_dependent_relayhost_maps = hash:/etc/postfix/relay_by_sender

If you use iRedMail, you can find the sender_dependent_relayhost_maps parameter and set the value to

sender_dependent_relayhost_maps =
    hash:/etc/postfix/relay_by_sender
    proxy:mysql:/etc/postfix/mysql/sender_dependent_relayhost_maps.cf

Lookup table can be a file, or in the form of MySQL/MariaDB database tables. Lookup tables will be searched in the specified order until a match is found.

Save and close the Postfix main configuration file. Next, we need to create the lookup table file.

sudo nano /etc/postfix/relay_by_sender

Add rules like below, this will make emails sent from [email protected] delivered via the relay host specified on the right side.

[email protected]           [smtp-relay.sendinblue.com]:587

Let’s say if you have a Linux server that hosts two websites and each website has its own mail server running on two separate hosts, then you can add the following two lines to make each website uses its own mail server.

@domain1.com                     [mail.domain1.com]:587
@domain2.com                     [mail.domain2.com]:587

Save and close the file. Then build the index file.

sudo postmap /etc/postfix/relay_by_sender

Restart Postfix for the changes to take effect.

sudo systemctl restart postfix

Set Up SMTP Authentication

Now we need to set up SMTP authentication so that the Postfix SMTP client can use the relay host. Edit the Postfix main configuration file.

sudo nano /etc/postfix/main.cf

Add the following lines at the end of this file. The /etc/postfix/sasl_password will contain the username and password.

# outbound relay configurations
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = may
header_size_limit = 4096000

If you have set the relayhost parameter, then I recommend giving it an empty value like below, because we are now using transport maps and sender dependent relayhost maps.

relayhost =

Save and close the file. Then create the /etc/postfix/sasl_passwd file.

sudo nano /etc/postfix/sasl_passwd

Add the SMTP relay host and SMTP credentials to this file like below. Replace smtp_username and smtp_password with your own username and password that are given by SendinBlue. Note there’s a colon between the username and password.

[smtp-relay.sendinblue.com]:587            smtp_username:smtp_password

Save and close the file. Then create the corresponding hash db file with postmap.

sudo postmap /etc/postfix/sasl_passwd

Now you should have a file /etc/postfix/sasl_passwd.db. Restart Postfix for the changes to take effect.

sudo systemctl restart postfix

By default, sasl_passwd and sasl_passwd.db file can be read by any user on the server.  Change the permission to 600 so only root can read and write to these two files.

sudo chmod 0600 /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db

Testing

Now you can send test email to the recipients defined in transport maps, or send an email from the address specified in sender dependent relayhost maps. Then check the mail log (/var/log/mail.log or /var/log/maillog) to see if it’s working.

Note that if you are using a third-party SMTP relay service like Sendinblue, then it’s likely that you are required to validate your domain name in your account and edit SPF and DKIM record.

Troubleshooting

error #1

If your email wasn’t delivered and you found the following message in the mail log (/var/log/maillog),

Relay access denied (in reply to RCPT TO command))

then you might need to edit the /etc/postfix/sasl_passwd file and remove the port number after the hostname like below.

smtp-relay.sendinblue.com    smtp_username:smtp_password

Save and close the file. Then build the index file again.

sudo postmap /etc/postfix/sasl_passwd

Restart Postfix for the changes to take effect.

sudo systemctl restart postfix

Now you can flush the email queue (attempt to deliver the previous emails).

sudo postqueue -f

error #2

Note that sender_dependent_relayhost_maps doesn’t support the special keywords: smtp, local, etc. If you see the following error in Postfix logs, it means you used the smtp keyword.

Host or domain name not found. Name service error for name=smtp type=A: Host not found

Bonus Tip: smtp_fallback_relay

You can specify a fallback relay host in Postfix. This way, if an SMTP destination can’t be reached (no MX record, no A record) or the primary relay host is offline, Postfix will use the fallback relay host.

Edit the main Postfix configuration file.

sudo nano /etc/postfix/main.cf

Add the following line in this file. Replace secondary.relayhost.com with the actual relayhost name. This relay host can be from Sendinblue or any other SMTP relay services.

smtp_fallback_relay = [secondary.relayhost.com]:587

Save and close the file. Then you should edit the /etc/postfix/sasl_passwd file and add the SMTP credentials for the fallback relay host just like above. Finally, restart Postfix for the changes to take effect.

Note that if the SMTP destination is using greylisting to temporarily reject email messages, then your Postfix SMTP server will also use the fallback relay host.

Per-User Relay Rules with Virtual Mailboxes

Let’s say you have set up business emails on a third-party service like Zoho. Now you have a self-hosted email server which is using virtual mailboxes. You want to keep the old email addresses on Zoho, but create new email addresses in your own email server. In this case, you need to create per-user relay rules so that emails destined for the old email addresses will be sent to the Zoho mail server.

Edit Postfix main configuration file.

sudo nano /etc/postfix/main.cf

Find the virtual_mailbox_maps parameter and add hash:/etc/postfix/transport to the value field.

virtual_mailbox_maps =
   hash:/etc/postfix/transport
   proxy:mysql:/etc/postfix/sql/mysql_virtual_mailbox_maps.cf,
   proxy:mysql:/etc/postfix/sql/mysql_virtual_alias_domain_mailbox_maps.cf

Save and close the file. Then in the /etc/postfix/transport file, you can define relay rules for the old email addresses like below.  (Note: You can also use the relay keyword instead of smtp. They are both correct in this case.)

[email protected]      smtp:[mx.zoho.com]:25
[email protected]      smtp:[mx.zoho.com]:25
[email protected]      smtp:[mx.zoho.com]:25

Save and close the file. Then build the hash table for the /etc/postfix/transport file.

sudo postmap /etc/postfix/transport

Then restart Postfix.

sudo systemctl restart postfix

Wrapping Up

I hope this tutorial helped you use Postfix transport maps and sender dependent relayhost maps. As always, if you found this post useful,  subscribe to our free newsletter to get more tips and tricks. Take care 🙂