setting up client exchange outlook account

I get the following error message when i try to setup outlook with an exchange server account;

“The action could not be completed. The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action.”

It worked perfectly about half year ago, and one day, I just can’t connect to the mail server using Outlook. the name can’t be solved. however, it’s perfectly OK to use OWA, with the same account and password. I don’t know why my name just can’t be solved when attempting to use office outlook. could anyone help?