nnmail-extra-headers

nnmail-extra-headers is a variable defined in `nnmail.el'.
Its value is

(To Newsgroups Cc)

Documentation:
Extra headers to parse.
In addition to the standard headers, these extra headers will be
included in NOV headers (and the like) when backends parse headers.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.