Proxy Server won’t allow outgoing email

I installed Proxy Server 2.0 on a Windows 2000 Server. I am not running Exhange Server. I set everything up according to Windows 2000 Server Companion. I received 5 pop accounts from our ISDN provider and set them up. The first day the computerscould send and receive emails. The next day the clients can receive but are no longer able to send. I get a “Server Response: 550 rejected: administrative prohibition (account: mail.serviceprovider.com, SMTP Server: mail.serviceprovider.com) Error0x800ccc79. There is only one server and no changes have been made since the install of Proxy Server. Everyone has been granted appropriate access but still no luck. Any suggestions? Thanks in advanced!