We are using MS Exchange Enterprise edition v5.5 with SP3 running WinNT Server 4.0 with SP6. We have also a SMTP server running MS Exchange IMC services to the internet via corporate network infrastructure.
Some, not all, of our users experienced duplicate or multiple e-mails from their internet counterparts. There are not many clues to resolve this problem from MS' websites. Do you have an answer? It's not a critcial problem but worst-affected users are complaining too much while we are trying to resolve their woes.
Its difficult to pinpoint the specific cause based on the information you have provided; however, I think I can point you in the right direction in terms of troubleshooting.
The first thing I would do is examine the headers of the duplicate messages. If the headers are identical in all respects, then something internal is causing the duplicate messages. If the headers differ in any respects, then something external is likely the cause. If the headers do not lead you to a successful resolution, then I would go down the following list of possible causes and see if any apply to you:
1. Are the affected users using the Microsoft Exchange transport or are you using the Internet Mail (POP3/SMTP) transport? If you are using Internet Mail, do you have the "Keep a copy of messages on the server" option de-selected?
2. Have you recently used the Move Server Wizard? If so, are the duplicate messages being sent to both a distribution list and one or more individuals on that DL? If so, this is a known issue with the Move Server Wizard that has yet to be corrected. If this is the case, the workaround would be to send email to the distribution list only and not to individuals, as well.
3. Are any of these users Microsoft Mail recipients? Messages sent to MS Mail recipients through the IMC can result in duplicate messages equal to the number of MS Mail recipients that are on the recipient list.
Also, have a look at the event logs on your Exchange server to see if any error or warning messages are being generated. Finally, if you have message tracking enabled, you can use it to see what route the messages are taking. This may also help to determine why and where the messages are being duplicated.
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