Why I'm Leaving Google Hangouts
github.com
github.com
In fact in Matrix we don't even have the concept of sending a message over federation - the only thing you can do is to synchronise the history datastructure.
We don't see Matrix as undermining XMPP: if you want decentralised conversation history then use Matrix. If you want fast stateless message passing, use XMPP. Infact, we're building an XMPP<->Matrix bridge, so that XMPP can federate with Matrix - so it's not like we're fragmenting things further. The point for Matrix is to try to defragment all the different protocols out there.
That's why I'm with Fastmail :)