tracing E-Mail back to Client PC with Exchange Logs

We have an old Exchange 2003 that did a lot of Mail-sending-without-boxes-through-software-stuff (SMTP without auth.). And shouldn't be doing it any more, but theres this one mailaddress that keeps sending e-mails through a wrong alias that gets rejected from exchange.
The Exchange ist still used for some other legacy stuff like public folders so we can't just decomission it.

So now i need to find out which piece of crapware on one computer in the network is responsible for this.
Sadly i can't seem to find the right screw that need tweeking in the exchange log-setting to tell me where the mail came from.
eSourceONEAsked:
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Alan HardistyCo-OwnerCommented:
You can use something like WireShark to sniff the network for port 25 traffic being sent to the server and then that should give you the sending IP address and that should identify the offending computer:

https://www.wireshark.org

Alan
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eSourceONEAuthor Commented:
Not quite the solution I had in mind but it did the trick.
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Alan HardistyCo-OwnerCommented:
As long as it worked and you found the culprit ;)

Thanks for the points.

Alan
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