A (multipart) MIME message can be converted to MML
with the mime-to-mml function. It works on the message in the
current buffer, and substitutes MML markup for MIME
boundaries. Non-textual parts do not have their contents in the buffer,
but instead have the contents in separate buffers that are referred to
from the MML tags.
An MML message can be converted back to MIME by the
mml-to-mime function.
These functions are in certain senses “lossy”—you will not get back
an identical message if you run mime-to-mml and then
mml-to-mime. Not only will trivial things like the order of the
headers differ, but the contents of the headers may also be different.
For instance, the original message may use base64 encoding on text,
while mml-to-mime may decide to use quoted-printable encoding, and
so on.
In essence, however, these two functions should be the inverse of each other. The resulting contents of the message should remain equivalent, if not identical.