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.
Ichabod is an SMTP/POP3 server, with mailing list support. Administration is via a web, or email
You can also use Ichabod to make your app receive email as an SMTP server.
You can also easily extend it by subclassing to add your own behaviors.