Microsoft Small Business Server was discontinued in 2013, leaving a real niche for MDaemon as lots of small businesses were forced to decide between "full blown" Exchange (as I like to call it) or the cloud and Office365. It's not a direct replacement - if you use all of Exchange's advanced features you'll most likely find something MDaemon won't do (shared custom category synchronisation springs to mind) and vice versa, but 95% of it's there. Some pretty competitive pricing and no database to go all skewiff at 4:55pm on a Friday afternoon make it an attractive proposition for smaller companies in particular.