Most mail transport software in the world uses a header format outlined in a RFC-822. RFC-822 however is only the greatest common denominator. In all these standards, the header consists of several lines, separated by newline characters. A line is made up of a field name, beginning in column one, and the field itself, offset by a colon and white space. The format and semantics of each field vary depending on the field name.
Fields can appear in any order.
Some however are optional, and may be added by the user. Others are added by the mail transport software. A complete zoo of formats is used here. Describes the content of the mail in a few words. At least that's what it should do.