rmail-mark-message
rmail-mark-message is a compiled Lisp function in `rmail.el
'.
(rmail-mark-message BUFFER MSGNUM-LIST ATTRIBUTE)
Give BUFFER's message number in MSGNUM-LIST the attribute ATTRIBUTE.
This is use in the send-actions for message buffers.
MSGNUM-LIST is a list of the form (MSGNUM)
which is an element of rmail-msgref-vector.