Kerio Connect is a great alternative to Microsoft Exchange. We have several clients running Kerio on Linux as their mail server, and we've even helped one by implementing a NetApp NFS export as the mail storage volume.
Since Kerio stores mail in a directory structure rather than as a monolithic database file, our scheduled snapshots of the mail store on the storage array represent instantaneous, brick-level backups. Pretty cool, if we do say so ourselves.
We look after Windows and Mac OS X clients as well as iPhones and Blackberry devices all accessing Kerio mail and calendars.
We've also recently performed some really beneficial duplicate cleansing on a Kerio mail store using Linux command-line utilities.