GNet Solutions » GNU/Linux Administration

Postfix has the ability relay emails through another mail server. This can be useful if you run a Postfix mail server in your local network and have a dynamic IP address (because most dynamic IP addresses are blacklisted today). By relaying your emails through another mail server that is hosted on a static IP address in a data center (e.g. your ISP's mailserver) you can prevent your emails from being categorized as spam.

To configure relaying on your Postfix mail server, you need a valid email account on another mail server.

In this example, I use smtp.gmail.com as the remote mail server on which I have a valid email account with:

• Username: someuser@gmail.com
• Password: SuperSecretPassword

I assume you have already installed Postfix as I won't go into the details of installing Postfix here.

Configure Postfix For Relaying¶

• To configure our Postfix server for relaying emails through smtp.gmail.com, run:
  

  postconf -e 'relayhost = smtp.gmail.com:587'
  postconf -e 'smtp_use_tls=yes'
  postconf -e 'smtp_sasl_auth_enable = yes'
  postconf -e 'smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd'
  postconf -e 'smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt'
  postconf -e 'smtp_sasl_security_options = noanonymous, noplaintext'
  postconf -e 'smtp_sasl_tls_security_options = noanonymous'
  

• Our username (someuser@gmail.com) and password (SuperSecretPassword) for smtp.gmail.com must be stored in /etc/postfix/sasl_passwd, therefore we do this:
  

  echo "smtp.gmail.com:587   someuser@gmail.com:SuperSecretPassword" > /etc/postfix/sasl_passwd
  

Lock Down SMTP credentials¶

• /etc/postfix/sasl_passwd must be owned by root, and no one else should have read access to that file, so we do this:
  

  chown root:root /etc/postfix/sasl_passwd
  chmod 600 /etc/postfix/sasl_passwd
  

• Now we must convert /etc/postfix/sasl_passwd into a format that Postfix can read:
  

  postmap /etc/postfix/sasl_passwd
  

This will create the file /etc/postfix/sasl_passwd.db.

• All that is left to do is restart Postfix:
  

  /etc/init.d/postfix restart
  

That's it. You can now test by sending emails over your mail server and having a look at your mail log. You should see that all your emails are now passed on to smtp.gmail.com (except the ones that have a local recipient, very useful for internal-only mail servers that still need to send mail to the internet).

Resources¶

• http://www.howtoforge.com/postfix_relaying_through_another_mailserver
• http://www.postfix.org/SASL_README.html
• http://mhawthorne.net/posts/postfix-configuring-gmail-as-relay.html