We were looking to replace our ancient Exchange server with an Open-Source equivalent but nothing has a feature-parity. Nothing has the widespread support as Exchange: ability to fetch on all mobiles, widespread desktop protocol support (not just IMAP but you know, that Exchangey binary protocol), a web interface etc. etc. etc.
It just is a massive continued success, although from my migration from 2003 to 2010 and subsequent pain to 2010 SP2, I didn't find it successful or painless - I don't miss maintaining it.
I think a lot of folks on here look at the world through very narrow glasses where they write Python web-apps under Linux, or work in SOHO environments that rely on external email providers; they don't come across the all-pervasive Windows desktop culture. They never need to touch Active Directory, let alone understand what it is or why they'd need it, and never have to touch an Exchange server or roll-out applications across an array of desktop machines.
It is a pity because some end up making short-sighted narrow-FOV comments about Microsoft and Windows etc. that are unnecessary. Just because we don't use a system, we shouldn't assume that nobody else does.