question about internet mail relaying:

I closed off our exchange 5.5 server from relaying UCE mail. well, almost. The trouble is, it doesn't relay the mail, but it still accepts it. at any given time, i can look at my outbound queue in the exchange server, and find about 30 emails to dubious destinations, with "<>" as the Originator.

What's the best way to stop my server from even accepting these emails for delivery? Do have have to setup SMTP authentication?

Thanks!