SMTP, the Simple Mail Transfer Protocol, is the protocol that moves electronic mail across the internet from the sending host to the receiving host. It was specified by Jon Postel of the USC Information Sciences Institute in RFC 821, published in August 1982. The document opens with a plain statement of purpose: “The objective of Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently.” That sentence has held up for more than four decades; SMTP is still the protocol that carries virtually every email message between servers.
The design is a simple command-and-reply dialogue over a reliable transport connection. A client opens a channel to a server and issues a small set of text commands; RFC 821 defines the core verbs that practitioners still recognize, including HELO to identify the sending host, MAIL FROM to declare the envelope sender, RCPT TO to name each recipient, and DATA to begin the message body. The server answers each command with a three-digit reply code. Because the protocol is line-oriented ASCII text, an engineer can speak SMTP by hand over a terminal connection, which made it easy to implement and debug across the many different systems on the early ARPANET and internet.
SMTP separates the envelope (the routing information exchanged in MAIL FROM and RCPT TO) from the message content (the headers and body sent after DATA). This separation lets a message pass through a chain of relays, each forwarding it one hop closer to the destination mailbox, which is how mail can be delivered even when the sender and recipient are never connected at the same time. The protocol was deliberately narrow: it transfers messages between hosts but leaves the format of the message itself to a companion standard, originally RFC 822.
A key limitation of the original protocol was that it assumed seven-bit ASCII text, which made it awkward for binary attachments and non-English character sets. The Multipurpose Internet Mail Extensions (MIME) later solved that problem at the message-format layer rather than changing SMTP itself, allowing rich content to travel inside the same envelope mechanism.
SMTP has been revised to reflect decades of operational experience. RFC 5321, published by John Klensin in October 2008, is the current core specification; it obsoletes RFC 821 and its intermediate successor RFC 2821, consolidating the mail-transport rules into one document while preserving the original command set and reply-code framework. The continuity is striking: a mail server written against Postel’s 1982 specification still speaks recognizably the same protocol as one written today.