I have some basic questions regarding SMTP connections, particularly for Postfix but would appreciate advice for MS Exchange too:
- When a single relay for a given remote domain becomes unavailable, how frequently does a postfix mta that is sending mail to that domain check back to see if the service is available?
- Are failed connections "remembered" (ie cached) at all?
- Does postfix ever check to see if "lower priority" relays are online while a "higher priority" relay is available?
- Are either of the above tunable in postfix?