Because it has a good suite of tools that work out of the box. There's LDAP, webmail, webDav/calDav/cardDav, admin interface, antivirus, anti-spam, etc. It integrates well with OwnCloud too which is a nice plus.

When I was looking to move away from gmail, my requirements where security and privacy and Kolabsys was one of the first services that popped up as a viable alternative.

The thing is, I was also looking for a reason to try out AWS and get back to using linux after years of powerpoint and archimate _poisoning_ so everything came together like that.

I highly recommend it, even if it took me about 5 working days to get spamassassin, openDKIM, ciphermail and TLS working for all components. But hey, now I get a 94% grade on www.emailsecuritygrader.com and a A+ on SSL Labs's test so hurray for me.