GNU Anubis is an outgoing mail processor. It goes between the MUA (Mail User
Agent) and the MTA (Mail Transport Agent), and can perform on the fly various
sorts of processing and conversion on the outgoing mail in accord with the
sender's specified rules, based on a highly configurable regular expressions
system. It operates as a proxy server, independently from mail user agents.
GNU Anubis can edit outgoing mail headers, encrypt and/or sign mail with the
GNU Privacy Guard, build secure SMTP tunnels (Simple Mail Transport Protocol)
using the TLS/SSL encryption even if your mail user agent doesn't support it,
or tunnel a connection through a SOCKS proxy server.

--------*      --------**      ------***
|  MUA  | ---> | Anubis | ---> |  MTA  |
---------      ----------      ---------

*   Mail User Agent (client)
**  An outgoing mail processor and the SMTP tunnel.
*** Mail Transport Agent (server)

WWW: http://www.gnu.org/software/anubis/