Winsock service works on workstation but not on server

I have developed a "mailing address quality" application that runs as a service and uses Winsock to communicate. Basically you send it a US address as a delimited string and it returns a USPS validated address as a delimited string.

When the service is installed on my Windows 2000 workstation it works great - multiple clients can connect and send addresses and get responses. I log new connections and address correction requests in the event log.

I install the same executable as a service on a Windows 2003 server and the communications fail. The server accepts the client connection request and the test client believes it is connected. The new connection is logged in the event log.

However, when the address to correct is sent, the service behaves as if it did not receive the address. Nothing is logged in the event log. I'm not seeing any connection errors.

I've checked to see if port filtering is set up on the server and it is not. Neither is the firewall. Could this be a security or policy issue? From a programming standpoint, I don't understand why it would work on a Workstation and not on the Server. Both are running the same framework versions and service packs. Any suggestions?