When is use the command on my cmd:
telnet smtp.gmail.com 465

The cmd just goes blank:

Thanks for any help.
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It isn’t blank – it is actually waiting for you to issue telnet commands. Press CTRL and ‘]’ key – it will take you to Telnet command window. Key in ‘h’ to find a list of telnet commands.
For specific SMTP commands follow commands under “SMTP transport example” in the wikipedia link.