Migrating mail systems is a nightmare, we're about to migrate from Scalix v10 to whatever the latest version of Exchange is. We've tried 2 tools sold for the job, neither worked for us. Looks like we'll be exporting PSTs and re-importing them, although Scalix doesn't export the message header info correctly, so more faffing...

Would have stayed with Scalix if we could find a decent support org, and the licensing costs weren't almost the same as MS now.

No help to you though.