The current standard defining the format of email message headers is RFC 5322. It specifies the Internet Message Format, including how header fields such as From, To, Date, Subject, and Message-ID are written and structured.
Published in October 2008, RFC 5322 replaced RFC 2822, which had replaced the much older RFC 822 from 1982. The newer standard updated the format to reflect modern email practice while preserving the basic header-and-body structure used by Internet mail.
RFC 5322 defines syntax, not the entire process of sending email. SMTP, specified separately, handles message transfer between mail servers, while MIME adds support for attachments and non-plain-text content. A common mix-up is choosing RFC 5321: that document concerns SMTP transport, not the syntax of message headers.
Headers consist of named fields followed by a colon and a value. They carry routing, addressing, identification, and descriptive information before the message body begins.