Emails sent from customer domain not being accepted by Exchange 2003

I have an issue where a particular customer is sending emails to one of our users on our Exchange server but they are not receiving them and the server is sending out an NDR. Other users on the same customer domain send emails out and are accepted by the same user .

What can I do to find out why this address is not being accepted? I have added their domain IP's to the Whitelist on exchange but still those emails are rejected.

How can I troubleshoot this issue?
TimSr. System AdminAsked:
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I would check out that use's OWA and make sure the person is not marked as "blocked sender".  I believe it was under options, manage junk lists.

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TimSr. System AdminAuthor Commented:
That was the first thing I checked and No he does not have that email blocked.
Well so much for the easy one.  You don't by any chance have the NDR?  It could be rejecting for any number of reasons.  When I get a call like this, I also head over to and see if their domain is part of a blacklist.  It could also be something like the attachment is too large.

What does the transport logs say?
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TimSr. System AdminAuthor Commented:
Haven't looked at those. Where will I find them? Also, is there any way for me to see the NDR or configure it so I get the NDR as well as the client? With the client saying they never receive the NDR it makes me wonder what is causing the error.
Please share the NDR
Will SzymkowskiSenior Solution ArchitectCommented:
Do the following...
- make sure that there are no transport rules that would be blocking emails going to this user
- make sure that the user "Sender" has the proper spelling of the name
- check the users mailbox properties to ensure that nothing has changed i.e. primarysmtpaddress

Mohammad Ishtyaq khatriSr. Systems EngineerCommented:
NDR from the sender would help identify the issue.
TimSr. System AdminAuthor Commented:
I never did get an solution to this issue so I am just awarding points because it kind of touched on the issue however I don't know if the solution I put in place, Added their server IP to the white list on the server, actually fixed the issue or not. In any case the user is now getting emails from the sender so I guess this issue is solved.
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