This user has SBS 2003 SP2, Exchange 2003 SP2, and Vista SP1 with Outlook 2007.
The mailbox store has no limits set and the values of the user object under the domain when viewing the data under adsiedit is <not set>
mDBOverHardQuotaLimit <not set>
mDBOverQuotaLimit <not set>
mDBStorageQuota <not set>
I have created a new Outlook user profile.
I have deleted the OST files and let Outlook recreate them.
I have even uninstalled Office and delted the Users/Username/AppData/Local/Microsoft/Outlook/ folders and then reinstalled MS Office.
None of the above steps fixed the problem.
I have noticed that it seems to throw the error when Outlook is updating the sent items folder. The user's mailbox is about 1.9GB
There are others running Vista and Outlook 2007 on this same SBS network without this issue, so it appears to be isolated to either this domain user or this PC.