I'm curious about all these comments about how much time it takes to maintain these systems. I've run a Debian+Courier+Spamassassin+Roundcube setup for over a decade for friends and family, and the maintenance required has been quite minimal. I have a script that emails me when security updates are available, and apt-get upgrade just takes a few seconds. Every couple of years I have to upgrade the Debian version, but that rarely takes more than a couple of hours. Outside of that, it just runs and runs.

Having said that, I am comfortable doing all that Linux sysadmin stuff, so maybe that's the difference. And if I were offering email accounts to people I didn't know, that would be an entirely different maintenance challenge.