Mail-in-a-box (https://mailinabox.email) on Ubuntu Server which installs Postfix, Dovecot, nsd, NextCloud, RoundCube, nginx, spam assasin, postgrey ... and a good admin panel.

Mine runs on an Intel NUC at home, with the a sensible ISP who naturally can provide a fixed IP address (everything open), IPv6, and clean address.

Been very happy with it, both the software, and the independence :) A little IT and network knowledge is useful if you're doing more than the standard (VPS) install, but that's to be expected.