Notes/Domino 4 and 5 Forum

Hello Everyone,
I have a strange issue. With the latest Mydoom virus outbreak some users have been receiving error messages bouncing back from other servers (even though the original messages were not sent from our clients-but I understand that the sender address was forged).
The problem that I am seeing is that the messages are being delivered to some inboxes even though they are addressed to a non-existing e-mail address.
Example: User Name - Mary Jonson, domain:acme.com
In the person document's user name field we have:
"User name" field:Mary Jonson/ACME/ACMEPARENT
Mary Jonson
"Short name and/or Internet address" field: mjonson
mjonson@acme.com

My question is: how is it possible that messages sent to mary@acme.com are being delivered to Mary's inbox if the address is nowhere in the configuration?

Thanks in advance,
Peter