SMTP - send an email from terminal

You can send an email using native SMTP commands while connecting to a mail server using Telnet connection. In the script below text in bold are the commands followed a space then free form string appropriate to the preceding command.

telnet mail_server.domain.com 25
220 mail_server.domain.com Microsoft ESMTP MAIL Service, Version: 7.5.7601.17514 ready at  Thu, 31 Mar 2016 13:53:42 +0100
helo
250 mail_server.domain.com Hello [172.31.1.1]
mail from: username@example.co.uk   <- this need to be valid address, otherwise you'll get:501 5.5.4 Invalid Address
250 2.1.0 username@example.co.uk....Sender OK 
rcpt to: username@example.co.uk
250 2.1.5 username@example.co.uk
data
354 Start mail input; end with <CRLF>.<CRLF>
from: Piotr
to: me
Subject: Test via telenet
              <- [enter] may also require a space
test body
test
              <- [enter] may also require a space
.  <-- the '.' dot is important as indicates EndOfMessage
250 2.6.0 <MAIL_SERkTgbLYluLCT0006cc2b@mail_server.domain.com> Queued mail for delivery
quit

Depends on your mail server the commands need to be in upper cases like HELO etc, the above was tested using Microsoft implementation.