message-ignored-news-headers

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

"^NNTP-Posting-Host:\\|^Xref:\\|^[BGF]cc:\\|^Resent-Fcc:\\|^X-Draft-From:\\|^X-Gnus-Agent-Meta-Information:\\|^X-Message-SMTP-Method:\\|^X-Gnus-Delayed:"

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

You can customize this variable.