From the Microsoft Exchange Team Blog:
Despite our attempts at documenting various break/fix scenarios, it can often be frustrating when trying to locate the information you need to resolve an issue as quickly as you would like. I decided to share this information in an attempt to help you resolve the majority of issues that can prevent your users from successfully logging on to Outlook Web Access.
Link: Troubleshooting Outlook Web Access Logon Failures in Exchange Server 2007
