We shouldn’t be sending mail on port 25 anyway. 587 is the “submission” port, for MUAs (which is what Discourse is) to submit mail to a fully-fledged MTA, using an encrypted and authenticated connection. AFAIK all decent ESPs support submission, and if a provider blocks 587 outbound, I’m more than happy to go full rage-womble on them, because there is absolutely zero reason to do so on anti-abuse grounds.