We have an ISA Server as a proxy (SP1, FP1, hotfix). Some external users who are working in our company try to
get their mails from the OWA server through the Internet, going through our ISA server.
The proxy authentication is solved over "protocol rules." "Allow" applies to all users who are in our Windows group. They sign on with their user information and the Internet works fine. When they log onto the OWA server, they are able to reach their inboxes, but when they try to open any mail from the inbox the OWA logon window reappears. Even when they sign on correctly, the windows reappears again and again until OWA writes "access denied." Sometimes the OWA logon window reappears before they even reach the inbox. When they are very lucky, they can read the content of one e-mail before the window reappears. In the "protocol rules," when I authenticate the users over their IP addresses, everything works perfectly. Please help. Thank you.
Are you by chance using Exchange 5.5? If so, you might be affected by the issue documented in OWA logon screen displays again when opening a mail message. It might also help to review the documentation for the ISA Feature Pack as it relates to Exchange. You can find details on protecting E-MAIL SERVERS and details on protecting Web and OWA servers.
Dig deeper on User Authentication for Microsoft Outlook and OWA
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