Postfix is the second (backup) mailserver, if the first registered one is not reachable (due to maintenance, line fault, other error) the second tightly carried server takes the messages and redirects them after the first one is reachable again.
In principle, only two steps are necessary:
in the public nameserver two MX entries are set, usually a primary and a secondary one
example.com. 86400 IN MX 10 mx1.example.com.
example.com. 86400 IN MX 20 mx2.example.com.
Three entries are added to the local postfix of mx2.example.com (main.cf):
Smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination
Relay_domains = $ mydestination, example.com
Alternatively, the e-mail addresses can be defined, that not all are accepted.