Did this ever get resolved. I am having the same problem. Installed the Scan Service software and getting the same message when trying to create a mailbox. The Xerox is being seen so it's got to be a communication issue and I'm sure you're on the right track with the firewall, but like you I do not want to turn off the firewall.
Xerox claims that the 6204 is Windows 7 compatable and this should mean all their software for it is as well. The latest version of the Scan software is not 64bit but is Windows 7 32bit.
I see that you got blown off by the Xerox forum rep with basically no answer.