Dovecot

Mail-in-a-Box lets you become your own mail service provider in a few easy steps. It’s sort of like making your own gmail, but one you control from top to bottom. Technically, Mail-in-a-Box turns a fresh cloud computer into a working mail server. But you don’t need to be a technology expert to set it up. Each Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software. It also includes contacts and calendar synchronization. The box also includes other standard mail functionality like spam protection (spam filtering and greylisting), mail filter rules, email client autoconfiguration, and automated backups to Amazon S3 and other services. The box also includes automatic DNS configuration when you let it become your nameserver so that it can set important DNS records for mail deliverability and security including SPF, DKIM, DMARC, and MTA-STS. When enabled, DNSSEC (with DANE TLSA) provides a higher level of protection.

Cyrus IMAP is an email, contacts and calendar server. Cyrus is free and open source. Features: Speed Security Efficiency Search CardDAV and CalDAV support Robust data storage Replication Scalable for large installations Flexible filtering support through Sieve Supports antivirus and antispam toolkits Handles deliverability: SPF, DKIM, DMARC Extensive authentication options, through Cyrus SASL JMAP support Long list of supported standards