Basics

SMTP is generally integrated within an email client application and is composed of four key components:

Local user or client-end utility known as the mail user agent (MUA)
Server known as mail submission agent (MSA)
Mail transfer agent (MTA)
Mail delivery agent (MDA)

SMTP works by initiating a session between the user and server, whereas MTA and MDA provide domain searching and local delivery services.

Email Addresses Case Sensitive?

In email address before the @, are case senstive.

Meaning Of Return-path, Reply-to And From?

From: <fromemail example.com="">
To: <you example.com="">
Reply-To: <replyto example.com="">
</replyto></you></fromemail>

Return-path: If email is failed to deliver OR bounced, then email will return to return-path.
Reply-to: If someone reply on email, then email will be goes to reply-to.
From: It denote from where which email-address came.

Commands

HELO – Identifies the client to the server, fully qualified domain name, only sent once per session
MAIL – Initiate a message transfer, fully qualified domain of originator
RCPT – Follows MAIL, identifies an addressee, typically the fully qualified name of the addressee and for multiple addressees use one RCPT for each addressee
DATA – send data line by line

References

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