You can use the telnet utility to do this. This utility is generally available on all available Windows, Linux, Mac OS operating systems.
Telnet is available in the terminal, so open the terminal to use it. For example, in Windows you can open a terminal by pressing the key combination “Windows + R” and in the field enter cmd and press Enter.
The utility is quite simple and to use it, all you need to know is the server name (domain name) or its IP and the port of the service you want to check.
For example, to check if the SMTP port is available, enter the following:
telnet mail.yourdomain.com 25
Otherwise you will get a connection error.
The cause of the connection error in most cases is the firewall, which can block the port on the client side,
as well as on the server side. Therefore, first of all, you should check the firewall settings on your PC.
If your PC does not have telnet installed, you can install it through the Control Panel.