User receives NDR report of email that he hasn't sent or not found in sent items. Exchange tracking and Mail GW show trace that user sent the mail.

I believe one of our users Email address has been spoofed because he is receiving non-deliverable messages of mails he hasn't sent or doesn't exist in his mailboxes "sent items". Although annoying I think that's pretty harmless.

However what feels alarming to me is that our messaging gateway shows the attempt to send such messages and I can even find them using Exchange 2010 Server tool Message Tracking.
What's common to these mails is that at the start of the subject line it says "Unread:" (or not read, I don't know the term in english because we have different locale) before the actual heading.
Should I be alarmed about this or is this expected behavior when someone receives NDR reports of emails that they have not sent.
Timo VAsked:
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Pete LongTechnical ConsultantCommented:
>> I can even find them using Exchange 2010 Server tool Message Tracking.

I'm assuming you mean the original message that produced the NDR? Check this users machine has no adware/viral infections on it.

P
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Michael B. SmithExchange & Active Directory ExpertCommented:
And if the user's machine is clean, you should be able to use message tracking to find the source IP address from whence the message originated.
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David Johnson, CD, MVPOwnerCommented:
one can save a reply to the folder that had the original message.  does the source IP point to the users computer?
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Timo VAuthor Commented:
I think this article describes what has happened.
https://www.simmonsconsulting.com/2008/10/26/my-outlook-is-sending-spam-but-not-really/

But still it feels weird that I can't find any trace of these emails being received by our mail gateway or Exchange server.

Symantec full scan didn't find anything and yes, the IP points to the users computer.
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Michael B. SmithExchange & Active Directory ExpertCommented:
Checking for viruses/malware and verifying the source IP are the two best places to start with this problem.
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