OWA – problem externally logging in

Hi,

We have SBS Windows 2003 Server, Exchange 2003 and all users use Windows XP.

Always seem like Monday is the worst day for problems. My users cannot access OWA externally. they type in the address https//xxxxx.xxxxxx.com/exchange and they get a PAGE CANNOT BE DISPLAYED.
Exchange is working fine – users are sending and receiving fine.
I tried to access OWA internally using the ip address which worked fine: – http://xxx.xxx.xxx.xxx/exchange.

Can anyone help please?

Thank you

Chris