Telnet is a useful tool that you can use to test the network connection to your server on a specific port. (The telnet protocol can do many other things as well.)
First, you will need to open a command prompt application.
You should now be looking at a command prompt, which looks like a box where you can type something. The general format of a telnet test is as follows. Type the following, then press Enter:
telnet example.com 25
Where EXAMPLE.COM is the domain or IP address of the server that you want to reach, and 25 is the port that you want to test.
A response of 220 or Connected indicates a successful connection to the server.
A connection timeout or response of Unable to connect indicates that your computer is not able to connect to the server on that port. This indicates that the port may not be open on the server, or that the network connection may be blocked.