Re:How to Run Your Own Mail Server (2017)
by z3t4 on Friday January 26, 2018 @05:30PM
(#16243444)
You can chain SMTP like this: SMTP (backup) -> SMTP (spam filter) -> MTA.
Checking white-lists can take a while and some SMTP servers will give up if they're stuck for over a second, sometimes less (eg when your server checks the spam-lists). Some SMTP servers or DNS server (cought Google caugh) will not try secondary though, so if you can afford it - have just one powerful front server with good uptime and network, or load balance / proxy to internal farm.
Checking white-lists can take a while and some SMTP servers will give up if they're stuck for over a second, sometimes less (eg when your server checks the spam-lists). Some SMTP servers or DNS server (cought Google caugh) will not try secondary though, so if you can afford it - have just one powerful front server with good uptime and network, or load balance / proxy to internal farm.