Btw, I ended up disabling webmail. I don't the users really need it. Nothing will compare to the Gmail experience anyway, so might as well just encourage people to use proper Mail clients like Mail.app or Mail on iOS.

Seconding for mailu. I've had a mailu server running for at least a couple of years that requires very little on-going maintenance, but I don't use it daily or for anything personally mission critical.

When I do need it, however, it's there, humming away happily.