Let’s start with SMTP. Simple Mail Transport Protocol. This is how e-mail gets sent. This is how e-mail makes it from you, to your recipient. When you check your e-mail, you use POP or IMAP to get the e-mail from the server. But when you’re sending e-mail, you use SMTP. SMTP is how your mail client communicates with your mail server, and then how your mail server communicates with other mail servers to deliver your precious e-mail to it’s destination.
SMTP has been around since 1982 and is used everywhere. It works, but it’s lacking in many ways, most of which are out of scope for this posting.
Basically the way it works is: