Configure Postfix as a backup MX

In order to configure postfix as a backup mx, you need to make some changes to the configuration file :

Add the following line to the main.cf file:

vi /etc/postfix/main.cf

Add  permit_mx_backup to the smtpd_restrictions :

smtpd_recipient_restrictions = current_restrictions, permit_mx_backup

permit_mx_backup_networks = x.x.x.x

Note : permit_mx_backup is not really necessary when you have the domains in relay_domains.

Now in order for the domains to act as a backup domain , these domains need to be added to the relay_domain option :

relay_domains = $mydestination domain1.com domain2.com domain3.com

Now the postfix will accept mails destined for these domains, now we need to tell it where to send these mails to , so we add transport_maps. Add the following to your main.cf :

transport_maps = hash:/etc/postfix/transport

When you make any changes to this file above, you will have to regenerate the hash by running the following command :

postmap /etc/postfix/transport

Sample transport file :

domain1.com smtp:mail.domain1.com domain2.com smtp:mail.domain2.com:2792

You can optionally define, how many days will the mails remain in queue if the primary mail server is down , by the following parameter :

maximal_queue_lifetime = 30d 

 

Only a minor note: permit_mx_backup is not needed when domains are listed in relay_domains. permit_mx_backup can be vulnerable to mis-use when access is not restricted with .