RFC 822, one of the most famous RFCs, documents the syntax of Internet mail headers. These headers contain normal, ASCII text that can be written and read by users. The end of the header is a single, empty line, so no blank lines are permitted in headers. Lines indented with whitespace are regarded as continuation lines, so long lines can be split "between lexical tokens". Header lines are formated as a field-name, followed by a colon, followed by field data. No whitespace may appear before the colon, and some field-names impose additional syntax requirements on their data fields. Common header fields are From:, To:, Cc:, Reply-To:, Subject: and Date:
RFC 822's address specification defines the syntax of an Internet email address. If you can muddle through the over-conceptionalized spec, you find that email addresses look mostly like this:
It is increasingly popular to lose the host portion, and let the network portion be a top-level DNS domain, writing email addresses more like email@example.com. This facility is implemented using DNS-based mail routing.
Privacy Enhanced Mail (PEM) is documented in RFC 1421, RFC 1422, RFC 1423, and RFC 1424. PEM defines standards for handling encrypted and authenticated mail, but has not been widely implemented, possibly due to questions regarding the patent status of its encryption algorithms.