So, I’ve spent some time recently working with Zarafa. It’s an exchange replacement that has a completely open source version as well as a version with licenses for outlook if you want that level of integration. It’s web interface looks good and is an improvement on Exchange server’s capabilities with a non-IE browser. Anyway it can install on a linux server and integrate with several different MTA’s. Our choice was postfix since that was already installed for the purposes of processing forms on a webserver on the same host. Outgoing mail “just worked”, but postfix needed a bit of configuration to make the incoming mail work.