Receiving email with attachment error coming from a specific domain

We have a customer that can send mail to our internal exchange server just fine but when they send us an email with any attachment (small in size), they get the following NDR...
The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk....

#550 4.4.7 QUEUE.Expired; message expired ##

Any ideas why this particular sender would be getting this reply?
Dennis JansonIT ManagerAsked:
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PradeepCommented:
If you are in blacklst automatically your mailflow will be effected.
First make sure that you are out of it and then try once.
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PradeepCommented:
Which version of exchange are you using..?
Are you able to do a telnet..?
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PradeepCommented:
Check this discussion once..
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Dennis JansonIT ManagerAuthor Commented:
I am running Exchange 2007 and I can telnet with no problem.
The discussion you posted does not help me.

I am listed on one blacklist TIOPAN and have been trying to get them to remove me for a while now without success. Some of these blacklist groups are terrible. It's a good service but only if it is maitained properly.
However I don;t think that this has any effect on this email domain issue. There is no materail in this NDR that states about blacklisting.
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Dennis JansonIT ManagerAuthor Commented:
I have been requesting TIOPAN to remove me from their list. It's been over a month and I've have not ever gotten a response.
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Sirreal45Commented:
Have you checked your message size limitations? is the attachment putting it over?
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Dennis JansonIT ManagerAuthor Commented:
I have set the attachment size up to 30 MB. It is not the issue in this instance.
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Sirreal45Commented:
What type of virus scanners are you using? If the user can send w/o an attachment, I don't think it's a blacklist issue.

can the sender send different types of attachments? .zip .doc etc, do they all get rejected?
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Dennis JansonIT ManagerAuthor Commented:
I am running a sonicwall firewall, mcafee client, and forefront security on the edge server. Usually I would get a message on my end stating if anything was filtered. I don't even see a record of the email with attachment even hitting our server.
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Dennis JansonIT ManagerAuthor Commented:
After the last blacklisting finally went away, I have not heard any further complaints about this issue.
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