Mailster is a project aimed at testing software mail capabilities. It provides a mail server container to test emails sent by your apps without rewriting your application code.
pop3serv is a RFC1939-compliant implementation of the server-side part of POP3, the protocol used to retrieve email from a server. pop3serv is a framework which allowes the easy creation of POP3 servers for lots of different purposes.
Virtual Qmail - virtual domains management system for qmail. It is designed to be fast, reliable, and easy to use. Includes software allowing users to manage their accounts via WWW.