Let me show you my scenario (very complicated, i guess)
I got a HW FW first, a Linux FW, as a second line of defense, and a Exchange 2003 SP2 FE. my BE is a Exchange 2000, last SP, on a W2K Server DC with the last SP. My organization borned as a Exchange 2000 Org.
All my messages are redirected from the Linux FW to the Exchange 2003. All messages are coming in and out without problems.
Before the arrive of Exchange 2003, the OWA service was placed with the Exchange 2000, without problems. Now, with the mail redirected to the new Exchange 2003, all users with the OWA functionality get the message “HTTP/1.1 503 Service Unavailable”. They use the same link as usual, “http://www.mydomain.com:8080/exchange”