[corford]

Zarafa is excellent (and works with MS Outlook if you need that).

Edit (to elaborate more): it's open source, has a very nice web ui, supports pop/imap/outlook, does activesync and mobile push, uses other standard opensource software (postfix, mysql), is easy to backup, comes with good documentation, is simple enough to get up and running (there are pre-built deb and rpm packages), integrates with LDAP and you can get a paid support contract if you need/want one. Their open source site is here: https://community.zarafa.com/

I'm not affiliated with them - just a happy sysadmin whose managed lots of Zarafa installs in the past.