Simple Mail Transfer Protocol

What does Simple Mail Transfer Protocol mean?

Simple Mail Transfer Protocol (SMTP) is an application-layer protocol used for transmitting electronic mail (email) messages between mail servers. It is a text-based protocol that uses the TCP/IP suite of protocols and operates on port 25.

SMTP is responsible for establishing a connection between the sending and receiving mail servers, initiating the email transfer, and ensuring its successful delivery. The protocol defines the commands and responses used to transmit email messages, including the sender’s and recipient’s addresses, email content, and attached files.

SMTP works by breaking down email messages into smaller units called “envelopes” and “bodies.” The envelope contains information about the sender, recipient, and routing instructions, while the body contains the actual email content. The sending mail server establishes a connection to the receiving mail server using the SMTP protocol and transmits the email message envelope first. After the envelope is successfully received and validated, the mail server transmits the body of the email message.

Applications

SMTP is a crucial technology in modern communication, enabling email to be sent and received across the globe. It is used by a wide range of applications, including:

• Email clients: Desktop and mobile email clients, such as Microsoft Outlook, Apple Mail, and Gmail, use SMTP to send email messages from users’ devices to mail servers.
• Webmail services: Web-based email services, such as Gmail, Yahoo Mail, and Outlook.com, rely on SMTP to transfer email messages between their servers and users’ browsers.
• Mail servers: SMTP is the primary protocol used by mail servers to exchange email messages with each other. Mail servers often act as intermediaries, receiving and forwarding email messages from and to different domains.
• Email gateways: Email gateways are devices or software that convert email messages between different protocols, such as SMTP and X.400. SMTP is commonly used as the outbound protocol for gateways.

History

SMTP was developed in the early 1980s as part of the TCP/IP suite of protocols. The first version, SMTP 1.0, was defined in 1982 in RFC 821. Subsequent versions, including SMTP 2.0 (1989, RFC 1123) and SMTP 3.0 (1998, RFC 2821), added support for new features and improved security.

SMTP 3.0 is the most commonly used version today. It introduced Key enhancements, such as support for Extended SMTP (ESMTP), which allows for additional commands and extensions to be added to the protocol. ESMTP is widely used for features such as email Authentication (SMTP AUTH), secure email transmission (TLS), and binary file transfer (MIME).

SMTP has evolved over the years to address challenges in email delivery, including Spam, phishing, and malware. SMTP filtering and anti-spam measures have been developed to help protect users from these threats.