If we make an analogy to physical mail, the SMTP commands MAIL FROM and RCPT TO address the envelope. Those commands give the SMTP server information on how the mail is to be delivered. In this analogy, the DATA command would be the letter inside the envelope. As it's common to address a physical letter inside an envelope, it's also common to repeat the delivery information in the email, even though it was already sent to the SMTP server through the MAIL and RCPT commands.
A simple email may look like the following:
From: Alice Doe <alice@example.net>
To: Bob Doe <bob@example.com>
Subject: Re: The Cake
Date: Fri, 03 May 2019 02:31:20 +0000
Hi Bob,
Do NOT forget to bring the cake!
Best,
Alice
The entire email is transmitted to an SMTP server following the DATA command. A single period on an otherwise blank line is transmitted to indicate...
