Wants the ability to listen on a Unix domain socket, and relay to an SMTP server on a Unix domain socket.
It looks like Net::Server may already have support for the listening part, so it would just be a matter of documenting the magic required to do this.
Specifying options like this in the config file seems to achieve the listening part.
==
# listen on a Unix domain socket
listen localhost
port /path/to/mysocket|unix
==