In addition to the text or body, a message has header fields which say who sent it, when, to whom, why, and so on. Some header fields, such as Date and Sender, are created automatically when you send the message. Others, such as the recipient names, must be specified by you in order to send the message properly.
Mail mode provides a few commands to help you edit some header fields, and some are preinitialized in the buffer automatically at times. You can insert and edit header fields using ordinary editing commands.
The line in the buffer that says
--text follows this line--
is a special delimiter that separates the headers you have specified from the text. Whatever follows this line is the text of the message; the headers precede it. The delimiter line itself does not appear in the message actually sent. The text used for the delimiter line is controlled by the variable mail-header-separator.
Here is an example of what the headers and text in the mail buffer might look like.
To: firstname.lastname@example.org CC: email@example.com, firstname.lastname@example.org Subject: The Emacs Manual --Text follows this line-- Please ignore this message.