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I administer my work's network and I encounter a problem I never got before.

I got a user (user@example.com) who sends email to on of our customers (user@customer.example.org). When user@example.com sends an email to @customer.example.org domain's, the email gets duplicated by the number of people who are supposed to receive the email :

Example : - 2 people in the TO field : they both get 2 copies, - 1 person in the TO field and 3 in CC : everyone gets 4 copies.

At @customer.example.org they only have the problem with email sent from user@example.com. When user@example.com sends email to other domains, the problem doesn't happen. When another user from my company send an email to @customer.example.org the problem doesn't happen.

It might not be super clear, so if you have any questions, i'll try to answer as best as I can.

Thanks :)

PS : I forgot to mention that we are using Outlook 2003.

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What MTA is your company using? What MTA is the receiving company using? –  Wesley Apr 17 '12 at 20:06
    
As far as I know, it's Exim on my side, but I have no idea on the receiving company –  Ebpo Apr 17 '12 at 20:19

2 Answers 2

Troubleshooting steps:

  1. Check your user's Outlook application to see what custom rules exist. Perhaps a rule is creating the extra email.
  2. Check your MTA (Exchange? Zimbra? Sendmail?) for any rules that might replicate mail. Document your setup thoroughly. Test it extensively. Why the fuss? Because your next step, assuming that you don't find the culprit, is to...
  3. Kindly request that the recipient's IT team inspect their own MTA and/or client's email application. Show them how you have thoroughly inspected your own systems and that it is highly unlikely to be your problem. Show detailed tcpdump recordings if you have to.

Here's your simple checklist. Print it out. =)

  1. It's your user's mail client
  2. It's your email server
  3. It's your recipient's email server
  4. It's your recipient's email client
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I'm checked for the Outlook App already and everything looks fine. I'm not sure how to check the MTA tho. I'm gonna have to do some search :) –  Ebpo Apr 17 '12 at 20:20

If you are using Exim, check the mainlog. It should show your message going out and any recipients that the message was delivered to. Records will have a message id so that you can locate all the activity for the message. Normally, Exim will do one delivery to the domain.

It helps to have a full set of duplicates to one recipient with headers attached. Then you can examine the received headers which will often pinpoint the server introducing the problem. A co-operative recipient will help a lot.

Different mail clients use different methods to forward messages with the full headers. As you only need the headers, using cut and past to copy the headers into a message is a workable solution.

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