Each GRP file stores the emails for that day for that folder. So you will noticed the inbox probably has a decent amount of these files. If you delete one of them, that's effectively delete that day's emails from the inbox. If you are running on enterprise this feature can help too, message archiving. http://help.smartertools.com/SmarterMail/v11/Default.aspx?qq=%2fsmartermail%2fv11%2ftopics%2fsystemadmin%2fsettings%2fstorage%2fmessagearchiving.aspx
You can probably work something out with the user that you may pull all the GRP's that are older than X and archive them yourself, putting them in a separate location and zipping it up to conserve space, up to you.