Solution: Here are three possible explanations for this behavior

#1: The users are checking their mail too often. Some users will set their mail client to check their mail every minute. This causes problems when the next check begins before the last one has ended. The index file will no longer be synchronized with the mailbox file. The way to fix this is for the user to delete the index file for their mail client on their computer and for the administrator to delete the index file from the user's directory on the server.

If the user has their mail client set to "leave messages on the server" then the number of messages in the mailbox will affect how long it takes to do a new mail check. We recommend setting the check timer to be at least 10 minutes. The user can manually force a check if needed (if they are sure the last check has finished).

#2: Outlook's code for UIDL caching is broken. Basically, a good mail client saves its UIDL cache until it has a full, error-free POP3 session in which no messages are reported and a clean QUIT is issued. Then it kills its cache and starts over.

Conversely, Outlook has a tendency to kill its cache during aborted POP3 sessions -- in other words, it connects to the server on some basic level, but then backs out of the transaction at some point, tries to pull itself together and see if it got any new messages, thinks there wasn't anything (new or old) waiting, resets the UIDL cache and you see duplicates again the next time you connect. Incidentally, that next time can be part of the same POP3 check.

#3: Another program may be checking the mailbox while the user is logged in. There are hundreds of free/shareware/ActiveX controls/Java applets that can check mailboxes for new messages. A quick check shows that ICQ, 3 different freeware utilities, AOL Instant Messenger, Yahoo and Netscape Notifier all start POP sessions to check for the new mail. All could cause the same results with either POP or IMAP or with mail left on the server. The best solution is not to use these utilities.

#4: It has been found if you are using message rules to process your e-mail when received has also caused related problems. You need to verify that you do not have duplicate rules for any email addresses that you have in your address book. If you have two rules going to the same address, you need to delete one of the two message rules.