Lotus Notes – The server is not responding

Hi Experts,
While sending a mail with my Lotus Notes 6.5.6 I have received an error message.
It says that the server is not responding. Have a look at the error message below.
I searched several ways to find the problem, but everything was alright. Have you any idea for the problem?
Please help me to solve the problem.
Thanks in advance.

Lotus Notes
The server is not responding. The server may be down, or you may be experiencing network problems. Contact your system administrator if this problem persists.
