I recently discovered the Wild Duck mailserver, which sounds great from their description:
WildDuck is a modern mail server software for IMAP and POP3. Modern being scalable, unicode-first and API-controlled.
Written from scratch, including web mail client (supporting OpenPGP via Mailvelope), using Node.js.
However it’s unclear to me what exactly it is – a replacement for Dovecot, or Postfix, or both?