mml-tweak-sexp-alist

mml-tweak-sexp-alist is a variable defined in `mml.el'.
Its value is

((mml-externalize-attachments . mml-tweak-externalize-attachments))

Documentation:
A list of (SEXP . FUNCTION) for tweaking MML parts.
SEXP is an s-expression. If the evaluation of SEXP is non-nil, FUNCTION
is called. FUNCTION is a Lisp function which is called with the MML
handle to tweak the part.