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This is bare-bones, original, primitive e-mail.
Install opensmtpd (or similar), then ncat or nc or netcat (this mysterious cat has many names).
Start the opensmtpd service, then use netcat to speak with the mail-daemon:
nc localhost 25
The computer should respond with code 220, which means 'I am listening'.
220 hex ESMTP OpenSMTPD
HELO gmail.com
You say HELO and say where you are coming from.
The smtpd will not check, so I am going to lie to it.
Mail servers are easily impressed, so it will be pleased to meet you.
250 hex Hello gmail.com [::1], pleased to meet you
MAIL FROM: <admin@gmail.com>
All the mail commands start with 4 bytes, because it's easier for admins to program. Tell the mail daemon who you are in this format.
250 2.0.0 Ok
Then tell it who you're sending to.
RCPT TO: <www@dmz.rs>
250 2.1.5 Destination address valid: Recipient ok
Finally, tell it that you want to send DATA.
DATA
354 Enter mail, end with "." on a line by itself
Subject: turn off server please
very urgent
.
250 2.0.0 73864a49 Message accepted for delivery
You will find the email under /var/spool or /var/mail or similar.
If unsure, just take a part of your email, like FRAGMENT="turn off server please", then grep for it:
sudo grep -r $FRAGMENT /var/spool/*