Is this not possible to do with SmarterMail in the future?  We have a client that handles a lot of CAD drawings through email and refuses or doesn't know about OneDrive links, and must use Outlook.  The problem is that the 50Gb limit for the OST, but adding the additional mailbox like Exchange would solve this and auto-archive on the server end.  This way he can set the Cached view to 6 months, but archive anything older than 1 year to the second box. We're probably going to lose this client to Exchange Online, and I only see this going to happen more and more.  The client has their own MSP who is also shooting for moving, because that is what everyone uses of course, so we are at a disadvantage there as well.