I have a mail server that operates a couple of gateways as a backup mx for a couple of clients (necessary due to their ISPs being very flaky), recently their Exchange server became corrupted (so the internet connectivity was still in place). During this time no messages were delivered to the backup gateway.
I've seen many circumstances where - if a mail server is playing up and not simply unavailable, sending servers will not attempt delivery to the backup mail server - does anyone have a definitive list of the circumstances? For example:
When sending server gets EHLO response but no delivery, backup mail server not contacted