"The primary goal of a backup machine for Postfix is
that if the first one fails, mail will be collected by the
secondary mail server until the primary mail server is back online.
When you build the Backup MX be sure to install all of the
necessary applications to scan for viruses and check for SPAM as
you do not want to overload the primary machine once it comes
"DNS Records for example.com would look like this:
IN MX 10 mail.example.com.
IN MX 20 mail2.example.com.
"In the illustration you can see that when the primary mail
server is unavailable, automatically the mail will be sent to
secondary machine which will hold all mail in the queue until the
primary server is online."