Function: metamail-buffer

Process current buffer through `metamail'.
Optional argument VIEWMODE specifies the value of the
EMACS_VIEW_MODE environment variable (defaulted to 1).
Optional argument BUFFER specifies a buffer to be filled (nil
means current).
Optional argument NODISPLAY non-nil means buffer is not
redisplayed as output is inserted.

(fn &optional VIEWMODE BUFFER NODISPLAY)