1) You can access from behind the firewall, correct?
2) Have you tried to access at least three diferent mailboxes of various levels? (admin, domain admin, domain user)
3) I'm eliminating permissions first. The account(s) you can access from behind ISA, can any of them access from the web?
4) Are you using a standard exchange log on path as the url? (http://www.company.com/exchange)
5) Can you hit your default web page? If you cannot, you probably missed something with ISA.
I don't think you're going to find this being an ISA issue. This will end up either IIS or Exchange...most likely Exchange since you can access behind the firewall, assuming your configurations are truly set up correctly on ISA.
Please post the results of your findings, they will better assist in the troubleshooting process. BTW, what version of Exchange?
David M. Streb, MCSE