Maybe its a great idea to implement the Zarafa Community edition.
It will add nice features like
- 3 Outlook users just like Exchange
- Webaccess with Outlook look&feel
- Mobile Sync (z-push @ sf.net)
Ofcouse you can send emails with imap & smtp.
Here is a demo
of the webaccess with sugarcrm & alfresco intergration.
I think this would be a pretty nice addon/module for ispconfig.