Simple Mail Transfer Protocol (SMTP)

SMTP is the protocol that makes email work. It is not concerned with what you write in an email — only with getting that email to the right place, much like the postal service handles an envelope without reading the letter inside.

The protocol relies entirely on the message header. Think of the header as the envelope: it carries the email addresses of all recipients. The body of the email — the actual message — is irrelevant to SMTP's job.

Here is how SMTP processes an outgoing email, step by step:

• When you send an email, the entire message (header + body) is placed into an outgoing mail queue on your mail server.
• A program called the SMTP sender picks up mails from this queue one by one and transmits them to the destination mail servers.
• As the SMTP sender successfully delivers a copy of the mail to a particular recipient, it removes that recipient's email address from the mail's destination list.
• Once the mail has been delivered to all recipients on the list, the entire mail is removed from the outgoing queue.

On the receiving end, a different program — the SMTP receiver — accepts each incoming mail and places it into the appropriate user mailbox on the destination server. …