I'm having trouble using the WiFi network at my school with my PowerBook and AirPort Extreme. The OS recognises the wireless network, and I have no trouble getting online, but the only types of network activity that seem to work are Rendezvous printing and internet browsers. Other things - Sherlock, Mail, Watson, iChat - just don't make it through. I assume this is due to some kind of firewall. Is there any way I can reconfigure my network settings so that I can use Mail at school the way I do at home?