If you have not already, you can download PuTTY from putty.org
Once you open PuTTY you can input the mail server name or IP address you are attempting to connect to into the Host Name field. Then update the port to 25 and the connection type to "Telnet".
When you click connect you should see something like this:
- If you do not receive a response, the connection was blocked by the mail server.
- If you receive "Cannot connect to host on port 25" the mail server is likely offline or is having network routing issues.
You can also run a few commands to test