Mailu
Insular email distribution - mail server as Docker images
...Security, enforced TLS, DANE, MTA-STS, Letsencrypt!, outgoing DKIM, anti-virus scanner, Snuffleupagus, block malicious attachments. Antispam, auto-learn, greylisting, DMARC and SPF, anti-spoofing. Freedom, all FOSS components, no tracker included.