This error appears when you try to move email from one folder to another, usually from the Inbox to an Archive (Personal Folder). If a user gets this error unexpectedly after a long stretch of things working fine, then chances are pretty good that they have filled up their PST file. Check their Data Files in their Outlook and see if the archive is an Outlook 97-2002 archive. These archives are more likely to fill up because their limit is 2GB. Newer PST formats have a default limit of 20GB but are configurable to greater than 20GB.
To work with or increase beyond the 2GB limit, either delete large messages from your archive, or create a second archive (in the newest PST format) and start storing email there (or move all the old stuff into it and delete the old archive).