Most laptop users use Outlook in “cached mode” while they are not connected to the Mail Server. Outlook will automatically synchronized with the Mail Server once it’s reconnected back to the network.
The Outlook AutoArchive is turned on by default, it will move items to the archive folder in a predetermined period. Old items will be deleted periodically. There may be conflict between the synch & archive functions.
I recommend turning off the AutoArchive function in Outlook. Instead, create a Personal Folders on the local drive in Outlook and archive manually.
If your users only connect to the Mail Server through OWA and want to archive the contents, you need to access it as an HTTP account. After connecting to the mailbox, the user can drag messages to a PST on your hard drive to archive them.
Note that when you access your mailbox using the HTTP protocol, you cannot use the special folders: Calendar, Contacts, Tasks, Journal.