This article aims to delve into how SMTP (Simple Mail Transfer Protocol) works and correct common misunderstandings about how SMTP servers "send and receive" emails. We will analyze in detail the complete transmission link of the email from the user agent to the final delivery, clarify the dual roles of server and client played by the Mail Transfer Agent (MTA) at different stages, and introduce the core SMTP commands, emphasizing that SMTP is only responsible for email transmission and not retrieval.

SMTP protocol core concepts

SMTP (Simple Mail Transfer Protocol) is the core protocol used for email transmission on the Internet. Its main responsibility is to transfer messages from the sender's mail client or mail server to the recipient's mail server. It is worth noting that the SMTP protocol itself only focuses on the "transmission" process of emails and does not involve the "retrieval" of emails. Retrieval of emails is usually handled by protocols such as IMAP (Internet Message Access Protocol) or POP3 (Post Office Protocol version 3).

For many beginners, there may be some misunderstandings about the concept of an SMTP server, especially regarding the responsibilities of "sending" and "receiving" mail. An SMTP server (usually referred to as a mail transfer agent MTA) plays different roles in the entire mail flow depending on the context - it can be a "server" that receives mail, or it can be a "client" that sends mail. Understanding this duality is the key to mastering the SMTP protocol.

Complete link for email transfer

The key to understanding the SMTP protocol is to master the complete life cycle of the email from the sender to the recipient. This process involves multiple components and protocols working together. The following is a simplified process:

1. Email submission (MUA -> MSA) :

   • When a user sends an email through a mail client (MUA, Mail User Agent, such as Outlook, Thunderbird, etc.), the MUA will submit the email to the Mail Submission Agent (MSA, Mail Submission Agent). The MSA is usually the server of the user's email service provider. It is responsible for receiving emails from the MUA and performing preliminary verification and processing. This phase also typically uses the SMTP protocol, but may be on a specific port (such as 587) and requires authentication.

2. Mail transfer (MSA -> MTA) :

   • After the MSA receives the email, it forwards it to the Mail Transfer Agent (MTA). The MTA is the core of the SMTP protocol and is responsible for the actual mail routing and transmission. In this step, the MSA connects to the MTA as an SMTP client.

3. Domain name resolution and routing (MTA -> target MX server) :

   • After the sender MTA receives the email, it will resolve the domain name of the recipient's email address (for example, example.com in recipient@example.com). It obtains the mail exchange record (MX record) of the domain name by querying the DNS (Domain Name System). The MX record points to the SMTP server responsible for receiving emails from this domain name (that is, the target MTA).
   • After obtaining the address of the target MX server, the sender's MTA will act as an SMTP client and actively initiate a connection with the target MX server to send the email. This is the key point to understand that an SMTP server "sends" mail: instead of pushing the mail directly to the user's inbox, it sends it to the next SMTP server responsible for processing the mail.

4. Mail reception and delivery (target MX server->MDA) :

   • After the target MX server (as an SMTP server ) receives the email, it processes the email and delivers it to the Mail Delivery Agent (MDA). MDA is responsible for placing the email in the storage area of ??the recipient's mailbox.

5. Mail storage and retrieval (MDA -> Mail storage -> MUA) :

   • MDA stores messages in a message store, which is typically served by an IMAP or POP3 server. When the recipient connects to the IMAP/POP3 server via its MUA, the message can be retrieved and read.

Key commands of SMTP protocol

Interaction with the SMTP protocol is mainly completed through a series of text commands. Among them, there are three core commands that form the skeleton of email transmission:

• MAIL FROM: : This command is used to specify the sender address of the email. This is not just the "From" displayed to the user, but the "Envelope From" used for internal processing within the mail system (e.g. bounce email addresses).
• RCPT TO:: This command is used to specify the recipient address of the email. An email can contain multiple RCPT TO commands, indicating multiple recipients.
• DATA : After specifying the sender and recipient, the DATA command tells the server the actual content of the email that will be sent next, including the email header (such as Subject, To, From, Date, etc.) and the email body. When the email content is transmitted, it usually ends with a single period (.) on a line.

After each command interaction, the SMTP server returns a three-digit status code, similar to the HTTP status code. These status codes indicate the execution results of the command (success, failure, more information required, etc.), and the client can decide the next step based on these response codes. For example, the 2xx series indicates success, and the 5xx series indicates permanent errors.

The difference between SMTP and email retrieval

Again, the SMTP protocol's sole responsibility is to implement email transmission. It does not provide any mechanism for users to "pull" mail from the server. Retrieval of mail is entirely handled by other protocols:

• POP3 (Post Office Protocol 3) : Usually used to download emails from a server to a local device. The copy of the emails on the server may be deleted after downloading.
• IMAP (Internet Message Access Protocol) : Allows users to manage mail on the server. Mail is usually retained on the server and users can access it synchronously on different devices.

Therefore, an "SMTP server" does not provide a mail retrieval interface, it only focuses on receiving and forwarding mail.

Summary and Notes

Understanding how the SMTP protocol really works is crucial to building or managing a mail system. The core is to realize that mail transmission is a multi-stage, multi-component collaborative process, and the MTA (SMTP server) plays a dual role in it: receiving mail as a server and sending mail as a client.

When developing mail-related applications, if the goal is to send mail, you need to connect to an MTA (as an SMTP client) and use commands such as MAIL FROM, RCPT TO, DATA, etc.; if the goal is to receive mail and store it locally, you need to implement an MTA to listen for SMTP connections (as an SMTP server) and process incoming mail. But please remember that this is only the transmission part. The final presentation and user interaction of the email also require the support of protocols such as IMAP/POP3.

Avoid confusing the SMTP server with the overall mail system that provides user mailbox services. The latter is a broader concept that includes the integration of multiple services and protocols such as SMTP, IMAP, POP3, and DNS. A thorough understanding of the division of responsibilities in these protocols will help us design and manage email systems more effectively.

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