Maddy Mail Server implements all functionality required to run an email server. It can send messages via SMTP (works as MTA), accept messages via SMTP (works as MX) and store messages while providing access to them via IMAP. In addition to that it implements auxiliary protocols that are mandatory to keep email reasonably secure (DKIM, SPF, DMARC, DANE, MTA-STS).
It replaces Postfix, Dovecot, OpenDKIM, OpenSPF, OpenDMARC and more with one daemon with uniform configuration and minimal maintenance cost.
IMAP storage is still in beta, but this is one to watch as it could dramatically simplify your infrastructure.