*Disclosure: I work on MongooseIM.*
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I recommend one of the Erlang-based servers: either MongooseIM or ejabberd. MongooseIM was originally a fork of ejabberd, but the codebases have diverged quite a bit in recent years.

These servers stand out among the other options not in terms of feature-richness ([they *do* offer lots of features, though](http://en.wikipedia.org/wiki/Comparison_of_XMPP_server_software)), but because they leverage the scalability mechanisms offered by the Erlang/OTP platform. To address your points:

> is scalable

Erlang is amazingly scalable. WhatsApp is the latest poster child for Erlang scalability, but if you google around, you'll find lots more.

> can handle between 10 and 15 million users

If you want to scale to millions of users, be prepared to build a cluster of beefy machines with lots of RAM (both these servers are memory-bound). There are deployments in the wild with ~2 million users that I know of. The [League of Legends](http://www.slideshare.net/michalptaszek/strange-loop-presentation) chat runs several federated clusters that handle 70 million users, but they're using a very heavily modified ejabberd.


> runs on Linux and/or Windows

You're probably better off running on Linux. Erlang does work on windows, but I'm not sure how well large-scale applications perform on the platform.

> is Open Source

Yes, both are open source, licensed under the GPL.