Message Logs - Track strange delivery issue...??

I have a user that send a message to a clinet and for some reason, it
also got sent to one of our global distrubution lists. (Really
strange)
I've looked through her sent item and it lists ONLY her as the sender
and him as the recipient.

I have a photocopy of the email from one of the people in the
distrubution group - in the TO: field it lists the client ("BILL SMITH
(bsmith@xxxxxxxxxx)") AND our global distrubution list
( <disList@xxxxxxxxxxx>)


Using the 'Message Tracking Centre', I can confirm that it DID get
sent to ALL the of the members of our distrubution group.

None of this makes any sense... how can I find out how this happened?

Thanks very much!
.