E-MailRelay is a simple SMTP proxy and store-and-forward message transfer
agent (MTA).  When running as a proxy all e-mail messages can be passed
through a user-defined program, such as a spam filter, which can drop, re-
address or edit messages as they pass through.  When running as a store-
and-forward MTA incoming messages are stored in a local spool directory,
and then forwarded to the next SMTP server on request.

E-MailRelay can also run as a POP3 server.  Messages received over SMTP
can be automatically dropped into several independent POP3 mailboxes.

WWW: http://emailrelay.sourceforge.net/