We built a large mail server cluster for BEL to handle email for around 7000 users. They had already been using GNU/Linux for their email for a long time (since 2005 in fact - we had proposed to do the setup even then!).
The server infrastructure there handles everything with full redundancy. All aspects of the solutions are locally hosted.
Abhas gave a short talk about “Bootstrapping a Free Software Business” at the Kerala Startup Mission in March 2015. Here is an overview: