postfix-users is active, and primary devs very frequently answer user questions (you should make an effort to go through the documentation first).

https://www.postfix.org/lists.html

To get an idea without subscribing:

https://www.mail-archive.com/[email protected]/

And thankfully the solution is almost always the same even after many years.

Choosing Postfix at this point in time is a mistake anyways though. Things like Stalwart or Haraka are just so much nicer to use.

Why? it's widely deployed, actively maintained and more importantly reliable. 10+ year old support threads are mostly still valid advice.

It needs a bunch of milters to have basic functionality, such as DKIM, SPF and DMARC. OpenDKIM and OpenDMARC are not well-maintained, reliable or nice to use.

Postfix was designed years ago, but that doesn't do it bad, it is a beast, it can handle 5k mails per second (DKIM signed) with an small tunning in a modern hardware without problem.

Postfix doesn't have DKIM support though. You'd have to use unmaintained OpenDKIM, no Ed25519, bunch of known bugs. It's not good. 5k/s is also really mediocre on modern hardware.