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RFC 5321 - Simple Mail Transfer Protocol

Published: October 2008
Status: Internet Standard (STD 10)
Author: J. Klensin
Obsoletes: RFC 2821
Updates: RFC 1123

Abstract​

This document is a specification of the basic protocol for Internet electronic mail transport. It consolidates, updates, and clarifies several previous documents, making all or parts of most of them obsolete. It covers the SMTP extension mechanisms and best practices for the contemporary Internet, but does not provide details about particular extensions. Although SMTP was designed as a mail transport and delivery protocol, this specification also contains information that is important to its use as a "mail submission" protocol for "split-UA" (User Agent) mail reading systems and mobile environments.

Table of Contents​

  • 1. Introduction
    • 1.1 Transport of Electronic Mail
    • 1.2 History and Context for This Document
    • 1.3 Document Conventions
  • 2. The SMTP Model
    • 2.1 Basic Structure
    • 2.2 The Extension Model
    • 2.3 SMTP Terminology
    • 2.4 General Syntax Principles and Transaction Model
  • 3. The SMTP Procedures: An Overview
    • 3.1 Session Initiation
    • 3.2 Client Initiation
    • 3.3 Mail Transactions
    • 3.4 Forwarding for Address Correction or Updating
    • 3.5 Commands for Debugging Addresses
    • 3.6 Relaying and Mail Routing
    • 3.7 Mail Gatewaying
    • 3.8 Terminating Sessions and Connections
    • 3.9 Mailing Lists and Aliases
  • 4. The SMTP Specifications
    • 4.1 SMTP Commands
    • 4.2 SMTP Replies
    • 4.3 Sequencing of Commands and Replies
    • 4.4 Trace Information
    • 4.5 Additional Implementation Issues
  • 5. Address Resolution and Mail Handling
    • 5.1 Locating the Target Host
    • 5.2 IPv6 and MX Records
  • 6. Problem Detection and Handling
    • 6.1 Reliable Delivery and Replies by Email
    • 6.2 Unwanted, Unsolicited, and "Attack" Messages
    • 6.3 Loop Detection
    • 6.4 Compensating for Irregularities
  • 7. Security Considerations
    • 7.1 Mail Security and Spoofing
    • 7.2 "Blind" Copies
    • 7.3 VRFY, EXPN, and Security
    • 7.4 Mail Rerouting Based on the 251 and 551 Response Codes
    • 7.5 Information Disclosure in Announcements
    • 7.6 Information Disclosure in Trace Fields
    • 7.7 Information Disclosure in Message Forwarding
    • 7.8 Resistance to Attacks
    • 7.9 Scope of Operation of SMTP Servers
  • 8. IANA Considerations
  • 9. Acknowledgments
  • 10. References
    • 10.1 Normative References
    • 10.2 Informative References

Appendices​

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