Exchange/.Brightmail Weirdness

We are seeing the strangest behavior in email flow.

We have a Sonicwall (TZ210)  not doing any anti-spam filtering. All email flow is directed to the Brightmail GW to have spam and nasties filtered out. The valid emails are sent to the Exchange server 2010 (SBS 2011). All spam filtering on Exchange is disabled.
Clients get their emails using Outlook 2007.

The issue: some emails do not get through. A user in domain abc.com, sends an email and even gets a delivery receipt. But recipient never receives the email. Another user in same domain abc.com sends emails without issues to the recipient.
So it is not the domain being filtered out as spam source. Something is randomly (so it seems) blocking emails from certain users.
We are also using Symantec Endpoint on all client computers.
Any help would be appreciated.
Thanks
OPCTechAsked:
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PostmasterCommented:
The mail logs from the Sonicwall and Brightmail devices should hold the answer. They will indicate if the mail has been rejected and why.
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Sudeep SharmaTechnical DesignerCommented:
One can always check the headers of the email to see the flow of the emails.

How to Extract Headers from Various Email Clients
http://mail.google.com/support/bin/answer.py?answer=22454

Message Header Analyzer:
http://mxtoolbox.com/EmailHeaders.aspx

I hope that would help

Sudeep
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OPCTechAuthor Commented:
I finally found the problem: Brightmail, by default deletes all emails containing unscannable attachments. Since some documents are unscannable by their very nature, a certain amount of apparently random emails were being junked.
I have adjusted the AV filter to allow unscannable documents

Thanks
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R. Andrew KoffronCommented:
This question has been classified as abandoned and is closed as part of the Cleanup Program. See the recommendation for more details.
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