Exchange 5.5 relay???

[Paladine]

I have an Exchange 5.5 server and I've set it up to require authentication to send mail. The problem is that I keep getting messages in the quee originating from <> to different e-mail addresses. I can't tell if these messages are getting relayed or not. It looks like the server can't resolve the host that they are trying to send to but I can't figure out how someone can send mail from <>. (A blank address). I know that you can just use the BCC field to hide your from address but how can it hide the originator from the server? And I would think that the mail should never get to the outgoing quee. (It should just dump the mail and sent the originator a "refused to relay" NDR. Any ideas????