SMTP (Simple Mail Transfer Protocol)

SMTP, the Simple Mail Transfer Protocol, is the protocol that moves email from one server to the next across the Internet. Jon Postel published the original specification as RFC 788 in November 1980 and refined it into RFC 821 in August 1982, the same month David Crocker's RFC 822 defined the message format. The two documents are complementary: RFC 821/5321 is the envelope (who is sending to whom, between servers) and RFC 822/5322 is the letter inside (the headers and body the user sees). SMTP is a line-oriented, text-based protocol. A client opens a TCP connection (port 25 for server-to-server relay, 587 for authenticated submission) and exchanges short ASCII commands — HELO/EHLO, MAIL FROM, RCPT TO, DATA — each met with a three-digit status code response. The DATA command introduces the RFC 5322-formatted message; the dot-on-a-line-by-itself convention marks its end. Every relay prepends a Received header so the trace chain accumulates as the message moves. The original protocol assumed strictly 7-bit ASCII transport, a limitation that MIME works around through Base64 and Quoted-Printable encoding. Later extensions added richer capabilities through the ESMTP framework introduced in RFC 1869: STARTTLS for opportunistic encryption (RFC 3207), 8BITMIME for negotiated 8-bit transport, SIZE limits, AUTH for client authentication, and PIPELINING for batched commands. RFC 5321 (John Klensin, October 2008) is the current consolidated specification.