I have recently received 3 reports of random non-existant Sent Items - the 3 individuals know they sent an email to the intended, in-house recipient (via the Exchange Server), but they each have noticed that a copy of their email is nowhere to be found in their PST file. Each intended recipient receives the email, which is verified by looking at their PST file. This problem is intermittent: other in-house emails being sent are being copied to the employee's Sent Items folder (in the PST file).
Our company has several clients running Outlook 2003, using Exchange 2000 (on Windows 2000 Server) with no POP3 email. Each client's primary store is a PST file on the server (because of Exchange's 16GB limitation), and auto-archiving is disabled for everyone. Everyone has "make a copy in the Sent Items" checked, and the 3 people that are currently having this issue do not have any filtering setup on their view. The problem appears to have started 2 or more weeks ago.