MIME issue: can't set encoding on mail or mml-to-mime

[Radu Butoi]

Hello all,

Following the message and MIME manuals, I want to change the
`Content-Transfer-Encoding` on outgoing mail to quoted-printable [1].
The "Encoding Customization" section mentions that these variables
control this setting:

(setq
 mm-content-transfer-encoding-defaults
 '((".*" quoted-printable))
 mm-body-charset-encoding-alist
 '((iso-8859-1 . quoted-printable)
   (utf-8      . quoted-printable))
 mm-use-ultra-safe-encoding t) [2]

However, this doesn't work and outgoing mail is not encoded. The
function quoted-printable-encode-region does seem to work, though. To
debug further, in the description for `mm-body-charset-encoding-alist`,
it mentions that encoding can me overridden through the encoding MML
tag. In a new buffer, I call M-: (mml-to-mime) on

<#part type=text/plain encoding=quoted-printable>
test test test <...longer than 78 columns...>
<#/part>

And this doesn't encode or set any encoding header. Setting encoding to
base64 doesn't change anything. I see this on Emacs 26.3, including with
-Q. I tried to edebug this but couldn't get very far. Is anyone familiar
with this library or can provide some pointers to help debug this?

Thank you,
Radu

[1]: This is in order to be able to send long, soft-wrapped lines for
Gmail to display plaintext emails properly on mobile -- I had previously
asked about this here:
https://lists.gnu.org/archive/html/help-gnu-emacs/2020-04/msg00216.html.

[2]: The documentation does state that `mm-use-ultra-safe-encoding` is
an internal variable not to be set; setting it or not makes no
difference, having tested both. Setting it to t does affect
quoted-printable-encode-region, as the description states.