In Emacs-26 manual(rmail.texi), some @code{CC} fixed to@{CC}.
But in rmail.texi, there still remain non-capitalized CC.
rmail.texi:756:@samp{Subject}, @samp{To}, @samp{CC}, @samp{In-reply-to} and
rmail.texi:759:sent the message you received, and the @samp{CC} field starts out with
rmail.texi:766:excluded from the @samp{CC} field. They are also excluded from the
rmail.texi:773:This omits the @samp{CC} field completely for a particular reply.
rmail.texi:942:against the @samp{To}, @samp{From}, and @samp{CC} headers (supply a prefix
rmail.texi:943:argument to exclude the @samp{CC} header).