[jcbrand]

Running an XMPP server is much easier than running a mail server.

I know, I've run both for a number of years.

Very few people need clustering support.

[jayd16]

My use cases were a large social app with chat and a AAA game with team chat. This was almost a decade ago but in both cases a single server was deemed insufficient by our ops people. I suppose you're right, though. I've never had to maintain a mail system of similar scale.

[gsich]

Mail is async in nature, XMPP has some expectancy to work as fast as possible (avoiding the term realtime here).