I cannot connect CRM to Outlook even though it is installed

I cannot connect the client CRM to Outlook. I asked a lot of people about this problem. The CRM has been installed successfully for Outlook for offline and online use. Everything else was working just fine but every time the connection is configured, the connection would fail and giving out the error:
“There is a problem communicating with the Microsoft Dynamics CRM server. The server might be unavailable. Try again later. If the problem persists, contact your system administrator.”
I have no idea what to do, so please help.

