SMTP

Simple Mail Transfer Protocol (SMTP)

The Simple Mail Transfer Protocol (SMTP) is a protocol used for sending email messages between servers. Most email systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an email client using either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP). SMTP is also generally used to send messages from a mail client to a mail server. This is why you need to specify both the incoming and outgoing mail servers when you configure your email application.

History

SMTP was first defined by RFC 821 in 1982, and it was last updated by RFC 5321 which includes the Extended SMTP (ESMTP) additions, and is the protocol in widespread use today. SMTP grew out of the need for a standardized method for sending email messages across different systems. Before SMTP, email systems were often proprietary and incompatible, making it difficult to send messages between different networks.

Basic Operation

SMTP operates over the Transmission Control Protocol (TCP), typically on port 25. The protocol follows a client-server model where the sending mail server acts as the client and the receiving mail server acts as the server. The process involves several steps:

1. **Connection Establishment**: The client establishes a TCP connection to the server. 2. **Mail Transfer**: The client sends the mail data to the server using a series of SMTP commands. 3. **Connection Termination**: The connection is closed once the mail transfer is complete.

SMTP commands include HELO, MAIL FROM, RCPT TO, DATA, and QUIT, among others. Each command initiates a specific action on the server, and the server responds with a status code and message.

SMTP Commands and Responses

SMTP commands are text-based and are sent from the client to the server. Each command is followed by a response from the server, which includes a status code and a message. Some of the most common SMTP commands include:

**HELO**: Initiates the SMTP session.
**MAIL FROM**: Specifies the sender's email address.
**RCPT TO**: Specifies the recipient's email address.
**DATA**: Indicates the start of the message data.
**QUIT**: Terminates the SMTP session.

The server responds to each command with a three-digit status code. Codes in the 200 range indicate success, codes in the 400 range indicate temporary failures, and codes in the 500 range indicate permanent failures.

Extended SMTP (ESMTP)

Extended SMTP (ESMTP) is an enhancement of the original SMTP protocol. It was introduced to provide additional functionality and to overcome some of the limitations of the original protocol. ESMTP is defined in RFC 1869 and subsequent RFCs. Some of the key features of ESMTP include:

**Authentication**: ESMTP supports various authentication mechanisms to verify the identity of the sender.
**Encryption**: ESMTP can use Transport Layer Security (TLS) to encrypt the communication between the client and the server.
**Enhanced Status Codes**: ESMTP provides more detailed status codes to indicate the result of each command.

Security Considerations

SMTP was not originally designed with security in mind, which has led to various vulnerabilities and issues over the years. Some of the key security concerns with SMTP include:

**Spam**: SMTP's lack of authentication and verification mechanisms makes it easy for spammers to send unsolicited email.
**Phishing**: Attackers can use SMTP to send fraudulent emails that appear to come from legitimate sources.
**Man-in-the-Middle Attacks**: Without encryption, SMTP communications can be intercepted and read by unauthorized parties.

To address these issues, various security measures have been introduced, including:

**SMTP Authentication (SMTP AUTH)**: Requires users to authenticate before sending email.
**TLS Encryption**: Encrypts the communication between the client and the server.
**DomainKeys Identified Mail (DKIM)**: Adds a digital signature to email messages to verify the sender's identity.
**Sender Policy Framework (SPF)**: Allows domain owners to specify which mail servers are authorized to send email on their behalf.

SMTP in Modern Email Systems

In modern email systems, SMTP is typically used in conjunction with other protocols and technologies to provide a complete email solution. For example, an email client might use SMTP to send outgoing messages and IMAP to retrieve incoming messages. Additionally, modern email systems often include various enhancements and extensions to improve performance, reliability, and security.