When you send and receive in Outlook, the personal folder or offline folders that you are delivering mail to may stop accepting new items. 

This problem can occur because of a modification in Outlook 2002 that limits the size of personal folder (.pst) and offline folder (.ost) files to less than 2 gigabytes (GBs) to prevent the files from becoming corrupted. Mail delivery to the file stops, but you do not receive an error message that indicates why. 


http://support.microsoft.com/kb/304863

http://office-outlook.com/outlook-forum/index.php?t=msg&th=36001/