You get an error that your mail server is not found/not responding, and everything else on your email client is configured properly. The most probable reason: port 25 is blocked by your ISP.
We do not block port 25 as it is needed for sending/receiving email.
However many Internet Service Providers do block port 25 in attempt to reduce spam via their mail servers. You can check if port 25 is available for your connection in the following way:
On a Windows System - start the command prompt:
Press Start-> run
Type "command" in the space provided
Now type "telnet yourdomain.com 25"
Wait a few seconds. If a message like the following appears, port 25 is open.
"220-serv01.clev.net ESMTP Exim 4.24 #1 Sat, 17 Apr 2004 03:17:36 -0500 220-We do not authorize the use of this system to transport unsolicited, 220 and/or bulk e-mail."
A message like "Connecting To yourdomain.com. ...Could not open connection to the host, on port 25 : Connect failed " means that port 25 is blocked. The reason might be either a firewall installed on your PC(more unlikely) or the firewall of your ISP(more likely).
On a Linux, UNIX or MAC - open a console and type "telnet yourdomain.com 25". If you receive a message beginning with "Connected to yourdomain.com", port 25 is open. Else port 25 is closed - the reason could be either a firewall installed on your PC or the firewall of your ISP.
In case port 25 is blocked by your ISP, you should contact them and ask them for the correct settings for the mail servers.