We’ll be using telnet to connect to a mail server and send ourselves an email.

The parts in bold are the commands to enter.

[bob@linux ~]$ telnet mail.website.com 25
Trying mail.website.com...
Connected to mail.website.com.
Escape character is '^]'.
220-mail.website.com ESMTP Exim 4.85 #2 Mon, 09 May 2022 22:12:59 
220-We do not authorize the use of this system to transport unsolicited, 
220 and/or bulk e-mail.
HELO domainto.sendfrom.com
250 mail.website.com Hello domainto.sendfrom.com [192.168.1.2]
MAIL FROM: <bob@incredigeek.com>
250 OK
RCPT TO: <bob@incredigeek.com>
250 Accepted
DATA
354 Enter message, ending with "." on a line by itself
Subject: Test Message

This is a test

.
250 OK id=5a1g7i-1347MT-1p
QUIT
221 mail.website.com closing connection
Connection closed by foreign host.

Further links to read

https://github.com/maildev/maildev/issues/212