Thanks, I was going to suggest this. The desktop mail client for OS X defaults to sending mail as quoted-printable. (Someday I'll transition away from mac mail and use mutt all the time, but not there yet)
This issue also included fixing the "From" addresses? Like in my case:
=?UTF-8?q?Fran=C3=A7ois=20Kooman?= <fkooman@tuxed.net>
I pushed a fix that addresses this for email bodies. I still need to address headers.
Fixed this for headers as well. This thread now shows correctly, for example:
https://lists.sr.ht/~sircmpwn/sr.ht-dev/%3C20180724165151.4353-1-fkooman%40tuxed.net%3E