That is a cool idea. I've typically run SmarterMail servers on the Windows side using a Storage Spaces backed array so that this happens automatically (that was their storage tiering system in previous builds of Windows) but there isn't a lot of control or visibility into what goes where. It would be neat to do something like that right inside SmarterMail. In the past I've seen people maintain two copies of a domain to accomplish this, with one of them running on SSD/flash and the other on platters:
With the users keeping their 20 year archives of email on the archive domain and anything recent on the domain itself. That might be a solution you could look at in the meantime.
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com