/Do not use this software!/
Metre (check that spelling) is an XMPP Server, of sorts. Unlike traditional XMPP servers which host services internally, Metre is specifically designed to connect between servers, mediating connections and traffic.
It's written in C++11 (ie, modern C++), and aims to provide a semantically-aware filter between domains, to avoid exposing your internal XMPP fully to the world.
It's also barely started. So don't use it unless you're a bleeding-edge trunk monkey who enjoys no security and loads of bugs.