Simple Mail Transfer Protocol (SMTP)

Email delivery over the Internet requires the following components:

1. Mail Submission Agent (MSA)
2. Mail Transfer Agent (MTA)
3. Mail Delivery Agent (MDA)
4. Mail User Agent (MUA)

The above four terms may look cryptic, but they are more straightforward than they appear. We will explain these terms using the figure below.

1. You (MUA) want to send postal mail.
2. The post office employee (MSA) checks the postal mail for any issues before your local post office (MTA) accepts it.
3. The local post office checks the mail destination and sends it to the post office (MTA) in the correct country.
4. The post office (MTA) delivers the mail to the recipient mailbox (MDA).
5. The recipient (MUA) regularly checks the mailbox for new mail. They notice the new mail, and they take it.

In the same way, we need to follow a protocol to communicate with an HTTP server, and we need to rely on email protocols to talk with an MTA and an MDA. The protocols are:

1. Simple Mail Transfer Protocol (SMTP)
2. Post Office Protocol version 3 (POP3) or Internet Message Access Protocol (IMAP)
   SMTP server listen port 25 by default

After helo, we issue mail from:, rcpt to: to indicate the sender and the recipient. When we send our email message, we issue the command data and type our message. We issue <CR><LF>.<CR><LF> (or Enter . Enter to put it in simpler terms). The SMTP server now queues the message.

Generally speaking, we don’t need to memorize SMTP commands. The console output above aims to help better explain what a typical mail client does when it uses SMTP.