message-ignored-mail-headers

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

"^\\([GF]cc\\|Resent-Fcc\\|Xref\\|X-Draft-From\\|X-Gnus-Agent-Meta-Information\\):"

Documentation:
*Regexp of headers to be removed unconditionally before mailing.

You can customize this variable.