Variable: mail-default-headers
mail-default-headers is a customizable variable defined in
sendmail.el.gz.
Value
nil
Documentation
A string containing header lines, to be inserted in outgoing messages.
It can contain newlines, and should end in one. It is inserted
before you edit the message, so you can edit or delete the lines.
If you are using message-mode to compose messages, customize the
variable message-default-mail-headers instead.
Source Code
;; Defined in /usr/src/emacs/lisp/mail/sendmail.el.gz
;;;###autoload
(defcustom mail-default-headers nil
"A string containing header lines, to be inserted in outgoing messages.
It can contain newlines, and should end in one. It is inserted
before you edit the message, so you can edit or delete the lines.
If you are using `message-mode' to compose messages, customize the
variable `message-default-mail-headers' instead."
:type '(choice (const nil) string))