So I've been reading up on ETRN, and configured it on my 2 Postfix servers - my primary and secondary MX.

But I can't see to find anywhere how to get the primary to connect to the secondary and issue the ETRN command automatically (eg, when Postfix starts). This seems to be the logical way this should work:

1) Primary server goes down
2) Secondary queues mail
3) Primary comes back up
4) Primary issues ETRN to Secondary to let it know that it can deliver mail to the Primary
5) Secondary acknowledges the ETRN and flushes the queue for the domains requested

Have I got the wrong understanding of ETRN? If it's a manual command, what is the point when I could just manually login to the secondary and run `postqueue -f`?

The way it works is this: when the primary MX goes down, the backup MXs queue the mail and keep trying to connect to the primary in order to offload the mail.
The primary may not know that it's unreachable (e.g. it may happen due to network problems), so if waiting for the backup MXs to contact the primary isn't
good enough, you might have to schedule a cron job. I haven't found anything in the documentation suggesting that postfix (in particular, the smtp client part of
it) can be configured to send an ETRN command, so you might have to write a simple script as suggested in this message from the mailing list.

Ah, see I wasn't sure if there was a way for postfix to do it internally. I guess not. A script in rc.local or modifying my init script will do the job nicely I think. Thanks for the link to that post