Event ID: 5504 logged in the DNS server

I am experiencing a significant number of Event ID: 5504 logged in my DNS server which is running Windows 2000 server and Exchange 2000. The description of the event is: “The DNS server encountered an invalid domain name in a packet from 192.203.230.10. The packet is rejected.

What does this event mean?
What is the cause of this event?
How can it be resolved and prevented from reoccuring?

Any insight is greatly appreciated.
Thanks, John Nelson