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. Also 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.