It would be great if it's able to verify accounts. You should use postfix setting "relay_recipient_maps" to query accounts against backup mx server.
if you can't verify account existence, just use postfix setting "relay_domains" is enough, all mails will be relayed to backup mx.
Sorry Zhang, I don't quite follow you...
in fact it would be better if the backup server would not check the recipient table. I explain with an example:
Let's say we have a domain with two MX:
mail.example.com PRIORITY 10
mail2.example.com PRIORITY 20
on the 'master' server, example.com has all the accounts configured (which can be modified, added, removed)
if the master server fails, the messages go on the backup server 'mail2.example.com' that has the 'mailBackupMX' set to YES.
I think it would be better for the backup server to just accept messages for any account for example.com, and the check on the recipient table is done on the master server when it comes back online and the queue is flushed from the backup server.
Is it possible to replicate such a behaviour?
I used to do it with Merak with a ETRN domain for the backup, and a cron job every 5 minutes on the master server with fetchmail using protocol ETRN. So there is no need to sync the accounts on the two servers...