I suggest stalwart mail, it can run in a binary or in docker, https://github.com/stalwartlabs/stalwart also +1 for mailcow.

that’s probably what I might do and build apks myself with forgejo. and/or pull in nel0x’s fork instead and build from his code.

Fair enough, I also recommend finding hetzner’s referral links to get some free credit initially when u sign up.

Not free, but if you can find them, https://www.scaleway.com/en/stardust-instances/ are pretty cost effective.