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.