Dithaka is a dynamic mailing list and collaboration engine written in Java. It merges the concepts of mailing list and web forum; every message sent to the list appears as a post in the forum, and every post to the forum generates a message to the list
CPO is a tool written in Java that manages newsletters. It's focused on corporate needs (privacy, scalability, bounce count, opt-in & out, newsletter statistics).
JEM is an SMTP server with pluggable components that let you
process email in any way you like. It can also extend beyond
emails into generalised message processing (like JMS) and can
even generate its own messages based on a Cron-like facility.