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 1423, and
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.