Sunday, September 9, 2012

Postfix configuration to use SMTP relay, re-write from email address and name

What is postfix?

Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers electronic mail. It is intended as a fast, easier-to-administer, and secure alternative to the widely used Sendmail MTA.

The postfix configuration file usually resides at /etc/postfix/ and uses the following format for configuration parameters.

parameter = value


You should set up a postmaster alias in the aliases table that directs mail to a human person. The postmaster address is required to exist, so that people can report mail delivery problems. While you're updating the aliases table, be sure to direct mail for the super-user to a human person too.

The aliases file is /etc/aliases. A sample configuration is shown below. In this configuration, the postmaster and root emails are delivered to the user john.

postmaster: john
root: john

Execute the command "newaliases" after changing the aliases file. Instead of /etc/aliases, your alias file may be located elsewhere. Use the command "postconf alias_maps" to find out.

Using an external SMTP server as a relay

By default, postfix directly delivers emails to the final email server. For example, if you send an email to, postfix will try to lookup the MX record for and directly connect to its mail server to deliver the email.

This can be a problem if your host cannot reach other email servers on the internet or your organization requires all emails to be sent via a SMTP gateway.

The parameter relayhost in the postfix configuration file is used for this purpose. Below is a sample configuration.

relayhost  =

Restart postfix using '/etc/init.d/postfix restart' for changes to take effect.

Re-writing the from-email address

If you want to re-write the email address from which emails are sent out from, postfix offers generic mapping for smtp. Create a file called '/etc/postfix/generic' and add in a line similar to the following one to rewrite the from address.

Modify the main configuration file to add the 'smtp_generic_maps' parameter as follows,

smtp_generic_maps = hash:/etc/postfix/generic

Then build the map generic db file using the command 'postmap /etc/postfix/generic'

Restart postfix using '/etc/init.d/postfix restart' for changes to take effect.

For any errors, check the log files located at /var/log/mail*

Re-writing the from name when sending an email

Although, the from email address is re-written using the process above, the name of the person in the email sent by postfix will still be shown as the local username. For e.g, if you re-write the email address from to and then you look at the raw headers in the sent email, the following may be seen.

From: root

To fix this, you need to use smtp_header_checks. This parameter allows you to have a regex replace the name used in the email header with the name of your choice. Follow the steps below to get this fixed.

1. Create a file called '/etc/postfix/smtp_header_checks'.
2. Add a regex in the file with the format and save the file.
          /^From:root/ REPLACE From: John
3. Add the following parameter to your mail configuration file.
         smtp_header_checks = regexp:/etc/postfix/smtp_header_checks
4. Check and compile your regex with a sample message using the command.
        postmap -q - regexp:/etc/postfix/smtp_header_checks < /tmp/raw-header-file
5. Restart postfix for changes to take effect.

More info

For more details and updated information, visit the postfix website at