I hazard a guess this is related to the changes in the with-colons
format of gnupg 2.1.15, since I just installed that to fix some other
breakage. I could be wrong though, I haven't debugged further than this.
If this message arrives signed, then a workaround is to set
mml2015-sign-with-sender to nil, although I'm not sure what will happen
with key selection.
> I have managed to duplicate (I think) this bug without notmuch or emacs
> In emacs 24.5.1 (on debian)
> (require 'mml2015)
> (setq mml2015-sign-with-sender t)
> M-x toggle-debug-on-error
> M-x message-mail
> (fill in some headers, From: me)
> C-c C-m C-s
> C-c C-c
I just tried this script in emacs 25.1 (debian) and could not reproduce
the bug. So maybe it's fixed?