RFC 2821:Simple Mail Transfer Protocol
RFC-Ref

Simple Mail Transfer Protocol


1. Introduction
2. The SMTP Model
2.1. Basic Structure
2.2. The Extension Model
2.2.1. Background
2.2.2. Definition and Registration of Extensions
2.3. Terminology
2.3.1. Mail Objects
2.3.2. Senders and Receivers
2.3.3. Mail Agents and Message Stores
2.3.4. Host
2.3.5. Domain
2.3.6. Buffer and State Table
2.3.7. Lines
2.3.8. Originator, Delivery, Relay, and Gateway Systems
2.3.9. Message Content and Mail Data
2.3.10. Mailbox and Address
2.3.11. Reply
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
3.5.1. Overview
3.5.2. VRFY Normal Response
3.5.3. Meaning of VRFY or EXPN Success Response
3.5.4. Semantics and Applications of EXPN
3.6. Domains
3.7. Relaying
3.8. Mail Gatewaying
3.8.1. Header Fields in Gatewaying
3.8.2. Received Lines in Gatewaying
3.8.3. Addresses in Gatewaying
3.8.4. Other Header Fields in Gatewaying
3.8.5. Envelopes in Gatewaying
3.9. Terminating Sessions and Connections
3.10. Mailing Lists and Aliases
3.10.1. Alias
3.10.2. List
4. The SMTP Specifications
4.1. SMTP Commands
4.1.1. Command Semantics and Syntax
4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO)
4.1.1.2. MAIL (MAIL)
4.1.1.3. RECIPIENT (RCPT)
4.1.1.4. DATA (DATA)
4.1.1.5. RESET (RSET)
4.1.1.6. VERIFY (VRFY)
4.1.1.7. EXPAND (EXPN)
4.1.1.8. HELP (HELP)
4.1.1.9. NOOP (NOOP)
4.1.1.10. QUIT (QUIT)
4.1.2. Command Argument Syntax
4.1.3. Address Literals
4.1.4. Order of Commands
4.1.5. Private-use Commands
4.2. SMTP Replies
4.2.1. Reply Code Severities and Theory
4.2.2. Reply Codes by Function Groups
4.2.3. Reply Codes in Numeric Order
4.2.4. Reply Code 502
4.2.5. Reply Codes After DATA and the Subsequent <CRLF>.<CRLF>
4.3. Sequencing of Commands and Replies
4.3.1. Sequencing Overview
4.3.2. Command-Reply Sequences
4.4. Trace Information
4.5. Additional Implementation Issues
4.5.1. Minimum Implementation
4.5.2. Transparency
4.5.3. Sizes and Timeouts
4.5.3.1. Size limits and minimums
4.5.3.2. Timeouts
4.5.4. Retry Strategies
4.5.4.1. Sending Strategy
4.5.4.2. Receiving Strategy
4.5.5. Messages with a null reverse-path
5. Address Resolution and Mail Handling
6. Problem Detection and Handling
6.1. Reliable Delivery and Replies by Email
6.2. Loop Detection
6.3. Compensating for Irregularities
7. Security Considerations
7.1. Mail Security and Spoofing
7.2. "Blind" Copies
7.3. VRFY, EXPN, and Security
7.4. Information Disclosure in Announcements
7.5. Information Disclosure in Trace Fields
7.6. Information Disclosure in Message Forwarding
7.7. Scope of Operation of SMTP Servers
8. IANA Considerations
9. References
10. Editor's Address
11. Acknowledgments
12. A. TCP Transport Service
13. B. Generating SMTP Commands from RFC 822 Headers
14. C. Source Routes
15. D Scenarios
15.1. D.1 A Typical SMTP Transaction Scenario
15.2. D.2 Aborted SMTP Transaction Scenario
15.3. D.3 Relayed Mail Scenario
15.4. D.4 Verifying and Sending Scenario
16. E. Other Gateway Issues
17. F. Deprecated Features of RFC 821
17.1. F.1 TURN
17.2. F.2 Source Routing
17.3. F.3 HELO
17.4. F.4 #-literals
17.5. F.5 Dates and Years
17.6. F.6 Sending versus Mailing
18. Full Copyright Statement
19. Acknowledgement

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