What does RFC 5321 define?

RFC 5321 defines SMTP (Simple Mail Transfer Protocol), the fundamental protocol for transmitting email across the internet. It specifies how mail servers establish connections, exchange commands, and transfer messages.

Key elements defined include SMTP command syntax (HELO, MAIL FROM, RCPT TO, DATA), response codes (250 for success, 4xx for temporary failures, 5xx for permanent failures), and connection handling procedures.

RFC 5321 also defines important concepts like the envelope (sender and recipients used for delivery) versus headers (displayed information), and relay behavior for routing messages through intermediary servers. Compliance with RFC 5321 is essential for reliable email transmission.

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