mm-use-ultra-safe-encoding

mm-use-ultra-safe-encoding is a variable defined in `mm-encode.el'.
Its value is

nil

Documentation:
If non-nil, use encodings aimed at Procrustean bed survival.

This means that textual parts are encoded as quoted-printable if they
contain lines longer than 76 characters or starting with "From " in
the body. Non-7bit encodings (8bit, binary) are generally disallowed.
This is to reduce the probability that a broken MTA or MDA changes the
message.

This variable should never be set directly, but bound before a call to
`mml-generate-mime' or similar functions.