SMTP (Simple Mail Transfer Protocol) Basics
SMTP (Simple Mail Transfer Protocol) is the standard communication protocol used for sending (outgoing) email messages from one server to another. It is utilized both when a user's email client sends a message and when mail servers exchange messages with each other.
1. Working Principle: The Three-Step Mail Transfer
SMTP is fundamentally a Push protocol, involving a three-stage process to deliver mail from the sender's server to the recipient's server.
- Client (Sender) → Sending SMTP Server: When a user clicks 'Send' in their email client (Outlook, Gmail web), the message is first transmitted to their configured Outgoing SMTP Server.
- Sending SMTP Server → Find Receiving Server: The sending server examines the email address's domain (e.g.,
naver.com), queries the DNS for the MX Record, and finds the address of the Receiving SMTP Server responsible for that domain. - Sending SMTP Server → Receiving SMTP Server Transfer: The sending server establishes a direct connection with the receiving server and transfers the email using a series of commands and responses. The receiving server then places the message in the recipient's mailbox.
2. Key Characteristics of SMTP
| Feature | Description |
|---|---|
| Push Protocol | The sender actively initiates the transfer and pushes the data to the recipient. (In contrast to IMAP/POP3 which are Pull protocols) |
| Text-Based | SMTP uses simple, text-based commands for communication (e.g., HELO, MAIL FROM, RCPT TO, DATA). |
| Port Numbers | It traditionally uses TCP Port 25, but for secure, encrypted transmission (SMTPS), it commonly uses TCP Port 465 or 587. |
| Security Improvement | Early SMTP lacked authentication, making it prone to spamming. Modern SMTP requires authentication (SMTP-AUTH) to verify the sender's identity. |
3. Summary of Email's Three Protocols
| Protocol | Role | Function |
|---|---|---|
| SMTP | Sending (Outgoing) | Transfers email between servers |
| IMAP | Receiving and Management | Accesses and synchronizes mail stored on the server |
| POP3 | Receiving and Download | Downloads mail from the server to the client |