Header shows more recipients than messagetracking / one recipient did not receive the mail

Environment:
(a single) local Exchange 2010 installation on Windows Server 2008 R2
AV: GFI MailEssentials 2012

Problem:
The message tracking on our Exchange only shows one of the recipients for an email (as opposed to 2 in the header info) (and that one with a different TLD than in the header, we use both .de and .eu though), i.e. one of the recipients seems to have been dropped somewhere along the way. I was gonna treat it as an isolated incident but we now had this happen twice with different senders (that we know of).

The header shows: john@contoso.eu; office@contoso.eu

and here's the tracking (john@contoso.eu is a distribution list with only the user jsmith@contoso.eu in it):
message trackingNow I realize that the header information is not reliable but nevertheless,  this was a legitimate email and the sender did initially send the mail to those two addresses in our organization and I need to troubleshoot this problem. Any ideas? Of course, I sent some test mails but the problem seems to occur sporadically.


( My initial question on this matter can be found here:
http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/Q_28366688.html )
sysadmin84Asked:
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Simon Butler (Sembee)ConsultantCommented:
Nothing has really changed from my original posting on the original question.
What the header says doesn't really matter. That doesn't prove the email was delivered correctly. If the message isn't in message tracking then Exchange didn't receive it. You could look at enabling logging on the Receive Connectors, then when the message comes in verify the connections.

As it is two different domains, I would also check the MX records are identical and the email isn't being sent to another server in error.

Simon.
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sysadmin84Author Commented:
Alright, logging is turned on, I'll report back if we're lucky enough to catch one of those mails again.
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sysadmin84Author Commented:
Update: So we had another case of this and this time I had the common sense to go into the mailbox and check the header itself. Turns out that the sender had a faulty autocomplete-entry (typically caused by Outlook falsely identifying a mailto: - link). Too bad the senders didn't contact their IT department or us about it since this produces an NDR.
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sysadmin84Author Commented:
Solved the problem.
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