Anubis is an outgoing mail processor, and provides the SMTP tunnel between
the MUA (Mail User Agent) and the MTA (Mail Transport Agent). It operates
as a proxy server daemon, independently from mail user agents, and can manage
and process outgoing mail on the fly using a highly configurable rule system,
based on a regular expressions (PerlRE) system. Anubis can edit outgoing mail
headers, encrypt and/or sign mail with GnuPG, build secure SMTP tunnels using
the TLS/SSL encryption even if your mail user agent doesn't support it, or
tunnel a connection through a SOCKS proxy server.

WWW: http://anubis.sourceforge.net/