Having to migrate two email servers this year, here's a summary of my options (might not be relevent to your needs) :
Distro : Debian 9 (raw)
MTA : Postfix + Dovecot
Antispam : Spamassassin
Antivirus : Amavis
Webmail : After Logic's WebMail Lite + Lighttpd + SQLite
Monitoring : Munin
Signing & such : SPF, DKIM, DMARC