SPAM Recipient question

We are looking into some spam that was received last night and are curious as to how it got around our network.

The email is a plain text message with what I assume is a fake From: address, the intersting part about this spam though is the recipient, Mail-User@ourdomain.com.  We don't have a mailbox or DL using this account but the mail was still received by about 50 users that we know of.

I've tried to sending a message to Mail-User@ourdomain.com but it comes back as not valid. Can anyone explain how this got to our users mailboxes?

We are using Exchange 5.5 SP4

thanx
burtco013Asked:
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v_alberCommented:
Message sample:
===========

To: Mail-User@ourdomain.com
CC:
BCC: all your domain users

Every-one will see Mail-User@ourdomain.com, but the message could be sent to all other people
0
SembeeCommented:
The user addresses were probably in the BCC. A false user was used to get round some of the anti-spam tools which reject email messages with no recipients.

Simon.
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burtco013Author Commented:
is there a way to find out who wa on the BCC list?
0
v_alberCommented:
Yes, if you have message tracking enabled, you can serach for specific message and where it landed
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burtco013Author Commented:
Thanx v_alber

we were able to track the message and found the BCC was to a DL on our server
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