I am working on a similar thing and used the approach of creating a resource called
teamshare@domain.com (this means a license is not used). For migrating the email I did originally attempt to use the Exchange tool; but due to how it appears to not take across HTML properly
Bug 44794 - Migrated messages w/ inline images from exch to zcs are not rendering I am now using Outlook to connect to both Zimbra and Exchange and drag 'n' drop the emails between the two. That approach works very nicely indeed
