message-forward-show-mml

message-forward-show-mml is a variable defined in `message.el'.
Its value is

best

Documentation:
*Non-nil means show forwarded messages as MML (decoded from MIME).
Otherwise, forwarded messages are unchanged.
Can also be the symbol `best' to indicate that MML should be
used, except when it is a bad idea to use MML. One example where
it is a bad idea is when forwarding a signed or encrypted
message, because converting MIME to MML would invalidate the
digital signature.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.