anubis

an SMTP message submission daemon
GNU Anubis is an intermediate layer between a mail user agent (MUA) and a mail
transport agent (MTA), receiving messages from the MUA, applying to them
a set of predefined changes and finally inserting modified messages into an MTA
routing network. The set of changes applied to a message is configurable on
a system-wide and per-user basis. The built-in configuration language used for
defining sets of changes allows for considerable flexibility and is easily
extensible.