Proxy and Exchange

Hi
Here are the facts.
1. Server=NT4.0 service 6a, exhange server 5, Formatted and reinstalled with no proxy service.
2. Workstations with 98. and outlook 98

Customer use to have a proxy server, and cleints with one modem shared, Now they are on a deticated ISDN, with Sonic Wall. The question is, when the Isdn is down, no one can receive their internal email, There is no external mail. As soon as the Web is back up, the email is normal, and does not lock up. They are using Outlook 98. I have uninstalled Proxy Cleint and looked through the registry for the client and found nothing, But 2 of the machines that has been rebuilt, does not have this problem. I do not want to rebuild 50 machines, or ghost them. Should be a simple answer ( Hope )
Thanks