message-followup command pops up a message buffer that’s a
followup to the message in the current buffer.
Message uses the normal methods to determine where followups are to go,
but you can change the behavior to suit your needs by fiddling with the
message-followup-to-function. It is used in the same way as
message-reply-to-function (see Reply).
message-use-followup-to variable says what to do about
Followup-To headers. If it is
use, always use the value.
If it is
ask (which is the default), ask whether to use the
value. If it is
t, use the value unless it is ‘poster’. If
nil, don’t use the value.