Dulplicate outgoing email from Mac, MS Exchange 2003

I have one employee who uses Mac mail client on his laptop connected to our exchange server 2003.

When he sends an email to a group, 3 recipients(two domains) receive duplicate emails every 10 minutes or similar interval. I checked Queue in Exchange server, the email bounded to the two persons were stuck and shows as 'Active' and '28 times failed'. But the recipients say they already received the same email multiple times.

Right now, I froze the outgoing email in queue so I can take a look at it more. I'm not sure if this is their domain email server problem or our email server or the Mac mail client. There seems some mail exchange communication problem between any of three.

To troubleshoot, what should I look at?
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So when he sent the email it keeps resending the same email over and over or do they receive the same email twice?
crcsupportAuthor Commented:
I looked at the email log, the email was recorded as it was sent out once, but the email was still on queue as 'Active' and it shows 28 times failed. And the recipient received more than 20 times. I guess the email reached the recipient email server, but my email server some how it thinks the recipient server didn't receive, some miscommunication between the two servers.
crcsupportAuthor Commented:
I still have the email frozen in queue, need to troubleshoot this. This happened a few times before recently.
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crcsupportAuthor Commented:
The delayed notification sent to the sender.

Reporting-MTA: dns;mail.xxxxxx.com
Received-From-MTA: dns;[]
Arrival-Date: Thu, 29 Mar 2012 23:42:48 -0400

Final-Recipient: rfc822;ccc@yyyyyyyyy.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 31 Mar 2012 23:42:48 -0400

Final-Recipient: rfc822;vvvv@yyyyyyyyyy.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 31 Mar 2012 23:42:48 -0400
It's possible it's stuck on his mac and his mac keeps resending the same email.

You can use the message tracking in under tools to confirm it.

Personally i would turn on the logging for smtp, clear the Queu and see if the same email re-appears and then run the message track on it and see it is coming from the mac or exchange.

The NDR is saying it is simply delayed which is normal for delayed email after a certain period of time.
I searched through EE and found this:

it is pretty much what I said above.

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crcsupportAuthor Commented:
I guess it was the apple email client putting too much header information, the receiving email server could n't handle it properly. I sent the same email using outlook, it goes through without duplicate email
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