It's a bit unconventional maybe, but I vote simple-nixos-mailserver
- IF you are curious / willing to learn nix. It's essentially just sanely configured dovecot, postfix, rspamd.
My config for those three combined is about 15 lines, and I have never had an issue with them. Slap on another 5-10 lines for Roundcube as a webmail client.
Since it's Nix, everything is declarative, so should SOMETHING happen to the server, you can be up and running again super quickly, with the exact same setup.