A secondary backup mail exchanger is a mail server which starts receiving mail once the main mail server goes down.

In Postfix, this is configured with the relay_domains and relay_recipient_maps as documented here. The configuration is fairly straightforward, although it does mean the backup MX needs to have an up to date list of valid recipients to avoid backscatter spam.

It is unclear how such a list should be constructed. Anarcat's uses /etc/passwd as a database of valid recipients, through the default configuration, namely local_recipient_maps= proxy:unix:passwd.byname $alias_maps. That list would need to be somehow extracted and copied over to the backup MX.

It may also be useful to change maximal_queue_lifetime specifically for that transport so that emails can survive for longer in case of outage. It is not possible to have a different expiry for the relayed destinations, see this FAQ for rationale.