‘Server is not a known TCPIP host’ is appearing in Lotus notes

I’m experiencing an error from the Lotus Notes client. The ‘server is not a known TCPIP host’ is always appearing and I can’t stop it. Anyway, I don’t know the meaning of it that’s why I can’t fix it on my own. Please help me explain the meaning of this and how to solve this. Thanks for your help guys.
