I've played with both Zimbra and Scalix and they seem quite nice and do pretty much what I want. I'm now at the point where I am considering retiring my trusty old courier-imap service in favour of one of these two, unless of course anyone has any other recommendations or suggestions. I'd probably be doing this inside a centos 4.4 Xen VMs running on centos 5 when it comes out.
I am pleased with the following setup, but first the features: 1. Sync with Palm (and others) 2. Instant/push email 3. Web client 4. Multi user 5. Open source / no limits (unfortunately, the palm software is not open source) 6. Active development
Software: 1. Postfix / Amavis new / sql grey / web-cyradm (spam, virus, etc) http://www.delouw.ch/linux/Postfix-Cyrus-Web-cyradm-HOWTO/html/index.html 2. Simon's Cyrus Imap (http://www.invoca.ch/pub/packages/cyrus-imapd/) including Idled for push email. 3. Egroupware (includes a SyncML server for syncing with mobile clients and a web interface). The web interface is a bit "open source looking," but works fine. (http://www.egroupware.org). I use the svn version so it is easy to update. (There is a feature for Cyrus Admin, but I have not used it). 4. Chatter email (palm client that supports idled for push email), costs money, works awesome (http://www.chatteremail.com/) 5. Synthesis SyncML Client (sync's contacts, calendars, todo's etc) with mobile client, costs money, works great(http://www.synthesis.ch/prod_client.php?lang=e&lay=desk)