RFC5321

RFC 5321 is the standard that defines the Simple Mail Transfer Protocol (SMTP) for transferring email between mail servers. Published in 2008 as STD 53, it supersedes RFC 2821 and, together with RFC 5322 (the Internet Message Format), constitutes the core of Internet email transport. The document specifies the client–server dialogue that underpins SMTP, including the concept of an envelope sender and envelope recipients separate from the header fields used by end users.

A typical SMTP session begins with a TCP connection, the client sending EHLO (or HELO) to announce its domain and to learn server capabilities. The server responds with a list of available extensions. The client then issues MAIL FROM to specify the envelope sender, RCPT TO for one or more recipients, and DATA to transmit the message content. The message lines are subject to a 1000-octet line length limit, and the body is terminated by a line containing only a period, with dot-stuffing applied as needed. Response codes follow a three-digit scheme, with 2xx indicating success, 4xx transient failures, and 5xx permanent failures.

SMTP supports extensions via EHLO, including 8BITMIME and SMTPUTF8, and, via other RFCs such as RFC 3207, can negotiate TLS with STARTTLS to provide encryption. Authentication is not defined by SMTP itself; SMTP servers often employ additional mechanisms (for example SASL) to authenticate both clients and relaying peers.

RFC 5321 is complemented by RFC 5322, which defines the syntax of Internet message headers and body, while together they govern the email transmission and format on the Internet.