message-default-headers

message-default-headers is a variable defined in `message.el'.
Its value is

""

Documentation:
Header lines to be inserted in outgoing messages.
This can be set to a string containing or a function returning
header lines to be inserted before you edit the message, so you
can edit or delete these lines. If set to a function, it is
called and its result is inserted.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.2 of Emacs.