Neal, unfortunately Message Archiving won't allow you to archive only the deleted items folders. Instead; Message Archiving allows you to set up rules for Inbound messages, outbound messages, or both. As such, you may need to use a more manual process in this case. I recommend reviewing the robocopy script below which can be used to take a 1:1 copy of whatever folder you desired, without locking critical files in SmarterMail. I recommend you set up a batch file to fire nightly that will call a robocopy command script for each user you want a backup of the Deleted Items for:
robocopy [source] [destination] /MIR /ZB /R:1 /W:3
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com