If you start with 'emacs -Q' and type ^x-m to compose mail, then use ^c-RET-p to insert text/plain part, then use ^c-RET-P (mml-preview) it will work.

It you wrap the text/plain part in multipart/alternative using ^c-RET-m, the mml-preview gives the error (Summary buffer not found).

On Wed, 28 Jul 2021 at 6:04 pm +0200, Lars Ingebrigtsen wrote:

Do you have a test case, starting from "emacs -Q"?