Setting Up Postfix As A Backup MX

1 Preliminary Note

I want to set up a backup MX for the domain example.com. In this example the primary MX for example.com is called mx1.example.com (IP address 1.2.3.4), so I call the backup MX mx2.example.com (IP address 1.2.3.5).

I have created MX records for example.com that look like this:

example.com.               86400   IN      MX      10 mx1.example.com.
example.com.               86400   IN      MX      20 mx2.example.com.

It's important that the primary MX has a lower number (10) and therefore a higher priority than the backup MX (20).

I'm assuming that the Postfix on mx2.example.com is already installed and working.

2 Configuring Postfix On mx2.example.com

To make mx2.example.com a backup MX for the domain example.com, all we have to do is change/add three lines to /etc/postfix/main.cf:

vi /etc/postfix/main.cf

First make sure that smtpd_recipient_restrictions contains permit_mynetworks and reject_unauth_destination, so something like this would be ok:

[...]
smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination
[...]

Then we must add example.com to the relay_domains paramater; if there's no relay_domains paramater yet in /etc/postfix/main.cf, the following will do:

[...]
relay_domains = $mydestination, example.com
[...]

And finally we add an empty relay_recipient_maps parameter to /etc/postfix/main.cf:

[...]
relay_recipient_maps =
[...]

(That way we don't have to specify a list of valid email addresses to back up, which might be a daunting task if you have to manage hundreds of email accounts.)

There's one important thing I have to add: You must not list example.com in the following parameters in /etc/postfix/main.cf:

• mydestination
• virtual_alias_domains
• virtual_mailbox_domains

That's it already. All we have to do now is restart Postfix: