SMTP is a protocol to send email over the internet and it is documented in RFC821. Sending email using Telnet is straight-forward if you know how to do it.
telnet smtp.example.com 25
MAIL FROM: <example_from @ example.com>
RCPT TO: <example_to @ example.com>
From: [John Doe] <example_from @ example.com>
To: [Jane Doe] <example_to @ example.com>
Subject: This is a test message….
This is an example email content to demonstrate email sending using Telnet.
Note that you need to type “.” and QUIT after you’ve finished writing the email content. You should change the SMTP server domain and the email used in the example accordingly.