Hi
As a proof of concept exercise, I did tweaked Zimbra 4.5 code to use Alfresco for mail (and all other items - calendar, documents) storage, instead of file system / MySQL tables.
It was not product-quality, with several bugs, and lots of untested things. It worked for Web interface and IMAP (SMTP not tested).
Also, I did not adjusted mail search to use alfresco search functionality.
But there were no business case for me at that time, so project was suspended.
If anyone is interested, I can publish code and we can look at it together.
Thanks
Alexander