Most mail transport software in the Unix world use a header format outlined in RFC-822. RFC-822 is only the lowest common denominator, however. 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. A header field can be continued across a newline if the next line begins with a whitespace character such as tab.
Fields can appear in any order.
Usually, all necessary header fields are generated by the mailer interface you use, like elm , pine , mush , or mailx . However, some 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. Multiple recipient addresses are separated by a comma.