What is Simple Mail Transfer Protocol (SMTP)?

Simple Mail Transfer Protocol (SMTP) is a foundational communication protocol used in the realm of email transmission. It serves as the underpinning mechanism for sending and routing electronic mail messages between email servers. SMTP operates within the application layer of the Internet Protocol (IP) suite, specifically using the Transmission Control Protocol (TCP) to ensure reliable and orderly data transmission.

Dissecting Simple Mail Transfer Protocol (SMTP)

SMTP, initially developed in the early 1980s as part of the ARPANET project, was a precursor to the modern internet. Jon Postel and Joy Reynolds, prominent figures in internet protocol development, were instrumental in its creation. The original SMTP was documented in RFC 788 in 1981 and later revised in RFC 5321 in 2008. It aimed to provide a standardized and efficient method for transmitting email messages across the emerging network of interconnected computers.

Key Phases in SMTP's Development

1. Original SMTP (RFC 788 - 1981): SMTP served as a foundational protocol for email transmission over ARPANET, offering a relatively simple approach for basic email communication.
2. RFC 821 (1982): Further refinements to SMTP came with RFC 821 in 1982, introducing essential improvements such as the EHLO (Extended Hello) command.
3. SMTP Extensions (RFC 1869 - 1995): Addressing evolving needs, RFC 1869 introduced SMTP extensions, including authentication mechanisms, to enhance security and reliability.
4. SMTP Service Extensions (ESMTP - RFC 5321 - 2008): The most substantial update to SMTP arrived with RFC 5321 in 2008, introducing Extended SMTP (ESMTP). ESMTP expanded the command set, enabling more efficient email transmission, enhanced error handling, and support for internationalized email addresses.
5. STARTTLS for Encryption (RFC 3207 - 2002): To address security concerns, RFC 3207 (2002) introduced the STARTTLS command, allowing SMTP to negotiate encryption using Transport Layer Security (TLS) or Secure Sockets Layer (SSL).
6. SMTP Authentication (SMTP AUTH): Authentication mechanisms like LOGIN and PLAIN were introduced to prevent unauthorized use of SMTP servers, enhancing security.
7. Submission Port (Port 587): Port 587 was designated for email submission, improving email sending practices and enhancing security, especially when used with SMTP AUTH and TLS/SSL.
8. Modernization and Continued Extensions: SMTP continues to evolve with modern extensions and updates, addressing contemporary email requirements, including anti-spam measures, email authentication standards (e.g., SPF, DKIM, DMARC), and IPv6 support.
9. Efforts to Combat Spam: Enhancements such as the "Sender Policy Framework" (SPF) and the "DomainKeys Identified Mail" (DKIM) authentication mechanisms have been introduced to combat spam.
10. Security and Privacy Improvements: Advances like "SMTP Strict Transport Security" (STS) have further strengthened SMTP, ensuring encrypted email transmission.

How SMTP works

To facilitate the transmission of email messages between email servers, SMTP uses a series of commands and responses to ensure that emails are delivered reliably and efficiently.

1. Sender's Email Client Initiates Communication: The SMTP process begins when a user sends an email using their email client (such as Outlook or Gmail). The email client establishes a connection to an SMTP server provided by the sender's email service provider. This connection is typically initiated over a network using Transmission Control Protocol (TCP).
2. Greeting and Handshake: Once the connection is established, the sender's email client initiates communication by sending a greeting command, usually "HELO" (or "EHLO" for extended capabilities). The SMTP server responds with a greeting of its own, along with information about its capabilities.
3. Sender and Recipient Addresses: The sender's email client provides the sender's email address (specified using the "MAIL FROM" command), and the recipient's email address (specified using the "RCPT TO" command). These commands allow the SMTP server to verify the authenticity of the sender and the existence of the recipient's email address.
4. Message Transmission: After recipient verification, the sender's email client transmits the email message using the "DATA" command. The email message, including its headers and body, is sent as plain text. The SMTP server receives and processes the message. It may perform tasks such as checking for message size limits, filtering spam, and applying security measures like TLS encryption if supported.
5. Message Queuing: If the recipient's email server is temporarily unavailable or busy, the SMTP server may queue the message for later delivery. This ensures that emails are not lost in transit, even in cases of network congestion or server downtime.
6. Acknowledgment and Delivery: Once the SMTP server successfully receives the entire email message, it responds with a "250 OK" message, indicating successful transmission. The SMTP server then takes responsibility for delivering the email to the recipient's email server. It may use DNS (Domain Name System) to determine the recipient's server's IP address.
7. Recipient's Email Server: The recipient's email server, which also runs an SMTP server, accepts the incoming message. It performs additional checks, such as virus scanning, spam filtering, and authentication. If everything checks out, the email is placed in the recipient's mailbox.
8. User Access: The recipient, using their email client or webmail interface, accesses their mailbox to retrieve the received email. This retrieval process is typically handled by email access protocols like POP3 or IMAP.
9. Optional Features: SMTP can support optional features like authentication, encryption using STARTTLS or SSL/TLS, and authentication mechanisms such as SMTP AUTH. These features enhance security and privacy during email transmission.
10. Error Handling: Throughout the SMTP exchange, both the sender's and recipient's servers communicate using numeric status codes to indicate the success or failure of each step. This allows for robust error handling and ensures that any issues are reported back to the sender for resolution.