Simple Mail Transfer Protocol

SMTP-transfer-model

Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (email) transmission. First defined by RFC 821 in 1982, it was last updated in 2008 with RFC 5321, which is the current version of the protocol. SMTP is used to send and receive email messages between mail servers and from email clients to mail servers.

History

SMTP was developed as part of the ARPANET project, which was the precursor to the modern Internet. It was designed to facilitate the transfer of email messages between different systems. The protocol has evolved over time, with various extensions and updates to improve its functionality and security.

Functionality

SMTP operates over the Transmission Control Protocol (TCP), typically using port number 25. The protocol follows a client-server model, where the client sends email messages to the server, which then forwards them to the recipient's mail server. SMTP uses a series of commands and responses to facilitate the transfer of messages.

Commands

Some of the key SMTP commands include:

• HELO or EHLO: Initiates the conversation between the client and server.
• MAIL FROM: Specifies the sender's email address.
• RCPT TO: Specifies the recipient's email address.
• DATA: Indicates the start of the message content.
• QUIT: Terminates the session.

Responses

SMTP servers respond to commands with numerical codes and text messages. Common response codes include:

• 250: Requested mail action okay, completed.
• 354: Start mail input; end with <CRLF>.<CRLF>.
• 421: Service not available, closing transmission channel.
• 550: Requested action not taken: mailbox unavailable.

Security

SMTP was not originally designed with security in mind, which has led to various vulnerabilities and issues such as spam and email spoofing. To address these concerns, several extensions and protocols have been developed, including:

• STARTTLS: Provides encryption for SMTP connections.
• SMTP Authentication (SMTP AUTH): Requires users to authenticate before sending email.
• DomainKeys Identified Mail (DKIM) and Sender Policy Framework (SPF): Help verify the authenticity of email messages.

Related Protocols

SMTP is often used in conjunction with other email protocols, such as:

• Post Office Protocol (POP3): Used by email clients to retrieve messages from a mail server.
• Internet Message Access Protocol (IMAP): Allows email clients to access and manage email messages on a mail server.

See Also

• Email
• Mail server
• Transmission Control Protocol
• Internet standard
• RFC 5321
• STARTTLS
• SMTP Authentication
• DomainKeys Identified Mail
• Sender Policy Framework
• Post Office Protocol
• Internet Message Access Protocol