popa3d is a POP3 daemon which attempts to be extremely secure, reliable, RFC compliant, and fast (in that order).

Dbmail is the name of a group of programs that enable the possiblilty of storing and retrieving mail messages from a database.

The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, webmail, and mailing list services within a single, consistent, framework. Individual components can be enabled or disabled at will. Courier now implements basic web-based calendaring and scheduling services integrated in the webmail module.