Can't logon to OWA through ISA 2006

[2624a]

Help, I configured OWA, it works fine internal. I then publish it on ISA Server 2006 for remote users but they are complaining that the HTML logon Form (Protected by ISA 2006) shows but when they type in the domain\username and the password, they can't logon to check there mail. The setting on ISA firewall Rule for OWA Authentication is Basic, Web listening on External, port 443.

Help......