Error at check name (Exchange unavailable)

I've upgraded from Outlook 2000 to 2003. I'm running Exchange server 5.5. The problem is that when you do the check name thing at the exhance section it doesnt verify it. It cant resolve or find the exchange server. Is there anybody that can help or knows what the problem is??? If so please reply via email to vanman@inetfreemail.co.za. I will really appreciate it and you never know - someday it will come back to you when you need some help. Thanks.