Hotmail will not send mail.

I have a network with a single server running MS Back Office Small Business Server 4.5 which includes Exchange 5.5 and Proxy 2.0. I am accessing the Internet though a WebRamp connected to an ISDN line. All workstations are running Win98. When a userattempts to send an email from her Hotmail or Yahoo Mail account,she gets an hour glass for about a minute and then a sceen that says:

Proxy Reports
10060 Connection timed out
The web server specified in your URL could not be contacted.

Other browsing works fine. If I log in as administrator, Hotmail works OK.