mail-yank-ignored-headers

mail-yank-ignored-headers is a variable defined in `sendmail.el'.
Its value is

"^\\(?:in-reply-to\\|m\\(?:ail-\\(?:f\\(?:ollowup-to\\|rom\\)\\|reply-to\\)\\|essage-id\\)\\|origin\\|re\\(?:ceived\\|mailed\\|turn-path\\)\\|s\\(?:tatus\\|u\\(?:bject\\|mmary-line\\)\\)\\|to\\|via\\|x-rmail-\\(?:\\(?:attribute\\|keyword\\)s\\)\\):"

Documentation:
Delete these headers from old message when it's inserted in a reply.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.