Configuring a Postfix backup server

Question

I have a subdomain subdomain.mydomain with two servers server1.subdomain.mydomain and server2.subdomain.mydomain running CentOS 5.5. server1 runs a Postfix server for subdomain.mydomain that acts as a second level SMTP server, i.e., it does relay through another server relay.mydomain (on a higher level domain mydomain).

I need to set up another Postfix on server2 to act as a backup server for server1, so that server2 relays to server1 all mail sent through server2.

I have been digging through the Postfix documentation site and found a section named Configuring Postfix as primary or backup MX host for a remote site, but it is too brief and not very clear.

How can I set up the Postfix service on server2 to relay to/backup server1?

What I have understood from the previous link to Postfix documentation is that I have to set up the following:

On server1 (main server)

• relay_domains must be equal to relay.mydomain
• transport_maps must be equal to hash:/etc/postfix/transport
• /etc/postfix/transport must contain the line subdomaing.mydomain relay:[relay.mydomain] (and brackets are mandatory for Postfix not to perform MX lookup)

On server2 (backup server)

• subdomain.mydomain must not be listed in mydestination
• smtpd_recipient_restrictions must be equal to permit_mynetworks reject_unauth_destination
• relay_recipient_maps must be equal to hash:/etc/postfix/relay_recipients
• /etc/postfix/relay_recipients must contain a line of the form user@subdomain.mydomain x for every mailbox user in the system

However, this does not work. When I start the Postfix server on server2 and connect using telnet, Postfix does not send any message (no welcome message and no response to any command).

Answer 1

If server2 is a backup to server1, why do you want it relaying messages to server1? The point of a backup is to provide service if the primary is down. So if server1 is down, server2 still needs to deliver.

For just a basic backup, server2 could be configured identically to server1. Then just create an MX record for server2, with server1's MX having a preference of 10 and server2's having a preference of 20.