Simple Mail Transfer Protocol (SMTP)

What is it?

What is it?

The "Simple Mail Transfer Protocol (SMTP)" is a standard protocol for sending and transmitting email messages between email servers over the Internet. SMTP governs email delivery by defining the rules and procedures for message transfer, addressing, and routing across networks.

Key points to remember

Key points to remember

  • Message Transfer: SMTP transfers email messages from the sender's mail server to the recipient's mail server by establishing a connection and transmitting the message content and metadata between servers.

  • Protocol Operations: SMTP defines a set of commands and responses that govern the interaction between email clients and servers during email transmission, including commands for initiating connections, sending emails, and handling errors.


  • Port 25: SMTP traditionally operates on port 25 for unencrypted communication or port 587 for encrypted communication (using STARTTLS). Mail servers use these ports to send and receive email messages over TCP/IP networks.


  • Relaying: SMTP servers can act as mail relays, forwarding email messages between domains or mail servers based on recipient addresses specified in the message headers. However, SMTP relaying may be subject to access controls and security restrictions to prevent abuse.


  • Authentication and Security: SMTP supports authentication mechanisms such as SMTP AUTH for verifying the identity of email senders and preventing unauthorized access to mail servers. Encryption technologies like TLS (Transport Layer Security) can secure SMTP communication and protect email content from interception or tampering.

Example of Use

Example of Use

  1. Email Transmission: An email client (e.g., Outlook) uses SMTP to connect to its designated SMTP server and transmit outgoing email messages, specifying the recipient's email address, message content, and other metadata for delivery.


  2. Mail Server Communication: When an email server receives an incoming email message from a sender's domain, it utilizes SMTP to communicate with the recipient's mail server, transferring the message and handling the necessary routing and delivery tasks.

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