Adds the functionality of consulting /mail/box/username/senders to see if marshall should override upasname/user depending on entries in the To list such that an entry in senders of
9fans@9fans.net mattmobile@proweb.co.uk
will force the sender to be mattmobile@proweb.co.uk whenever 9fans@9fans.net is in the To list (which also provides the motivation for doing it).