Postfix Backup MX

Reference: [1]

Backup MX

Setting up Postfix to function as a Backup MX is rather easy (assuming you already have it configured to function as a Primary MX)

You need only add the domain you want to buffer mail for to $relay_domains

relay_domains = $mydestination, domain_to_backup.com

Primary MX

Again assuming Postfix is already setup to finction as a Primary MX there are no additional modifications needed to receive mail from the Backup MX.

DNS MX Records

For the Backup MX to function your DNS must be setup correctly. DNS Configuration is beyond the scope of this article, but essentially you need to add an MX record to your DNS server for the Backup MX.

Example MX Record

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

Where mx1 is the Primary and mx2 is the Backup
The number 86400 is the Time to Live value (86400 seconds = 1 day)
The number 10 and 20 correspond to the MX's Priority

Notes

If you are using helo_checks (or any other checks for that matter) make sure you are allowing access from your Backup MX