It is vital that header composition follows a clear and unambiguous specification or format, to allow for parsing.
E-mail header: The text (body) is preceded by header lines indicating sender, recipient, subject, sending time stamp, receiving time stamps of all intermediate and the final mail transfer agents, and much more.
In a data packet sent via the Internet, the data (payload) are preceded by header information such as the sender's and the recipient's IP addresses, the protocol governing the format of the payload and several other formats. The header's format is specified in the Internet Protocol.
^P. Resnick, ed. (October 2008), Internet Message Format, Network Working Group, RFC5322
^B. Leiba (March 2013), "Update to Internet Message Format to Allow Group Syntax in the "From:" and "Sender:" Header Fields", Ietf Request for Comments (RFC) Pages - Test, Internet Engineering Task Force (IETF), ISSN 2070-1721, RFC6854