SMTP Protocol Explained (How Email Works?)


SMTP Protocol Explained (How Email Works?)

    Ever wondered what happens after you compose an email to your friend and then click the “send” button? How does the email end up at your friend’s mailbox?

In this article, we going to explain you step-by-step the full journey of an email message starting from when you click the “send” button until it lands in your friend’s mailbox. To understand how email works, you need to understand an internet protocol called SMTP or “simple mail transfer protocol”. for more tech notes, job updates, career tips follow our telegram channel Open2hire

SMTP protocol

What is SMTP?

  • SMTP is an Internet standard for sending emails on the internet.
  • If you feel adventurous and you want to actually read the specifications of the protocol, the original specifications for SMTP were publihed under RFC 821 in 1982. And later on, more specifications were introduced under RFC 5321.
  • I have to warn you though, these RFCs are very dry and, well, kind of boring to read.
  • Now instead of me going through the RFC, I’d rather explain how email and SMTP work by walking through a practical example.

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The Email Journey

Now let's take scenario, let’s follow the email message as it travels from Jhon to Wesley at a high level.

1- Jhon opens his Mail app, provides Wesley e-mail address (, writes his message, and clicks the “send” button

2- The Mail app starts communicating with Jhon’s mail server and eventually push the email that Jhon composed to Jhon’s mail server where it is stored to be delivered later to

3- Jhon’s mail server sees that there is a message pending delivery to It starts a communication with the mail server to allow for this message delivery to happen. It is here where the SMTP protocol comes into play. SMTP is the protocol that governs the communication between these two mail servers. In our particular scenario, Jhon’s mail server will play the role of an SMTP client while Wesley’s mail server will play the role of an SMTP server.

4- After some initial SMTP handshaking between the gmail and yahoo mail servers, the SMTP client sends Jhon’s message to Wesley’s mail server.

5- Wesley’s mail server receives the message and stores it in his mailbox so that he can read it later.

6- At some point, Wesley uses his Microsoft Outlook to fetch messages from his mailbox and eventually reads Jhon’s message.

Complete work flow PDF link in comment section



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