Matthew Lear <matt@bubblegen.co.uk> writes:

> Hi,
> I switched to trying emacs 25 (25.1) with notmuch the other day.

I'm on 25.1 as well, but haven't experienced the symptoms you describe.

>> !!! Bodypart handler `notmuch-show-insert-part-text/html' threw an error:
>> !!! Window is dedicated to ‘*unsent mail to user@domain.com*’
>
> Errors like these are generated by replying from either notmuch-tree or
> notmuch-show.

Below I tried replying from notmuch-show.

> I'm of the opinion that this is probably some sort of set up /
> configuration issue specific to 25, but I'm quite surprised. I don't
> have many settings for handling html stuff:
>
> (setq mm-text-html-renderer 'shr)
> (setq shr-inhibit-images nil)
> (setq mime-view-text/html-previewer 'shr)
> (setq mm-inline-text-html-with-images t)
> (setq notmuch-multipart/alternative-discouraged '("text/plain"))
>
> Is this reproducible for anyone else with notmuch from git/master as of
> today?

Matt, I couldn't resist helping another, Matt, so I copy-pasted your
setq incantations, executed them (thus "poisoning" my precious Emacs!),
and was able to initiate replies to several emails.  Some were
multipart/mixed with HTML alternatives, some (like yours) were
text/plain.

I'm running recent-ish notmuch binaries from git/master, running even
fresher elisp from git/master (I think I pulled last night).

It may be necessary for you to play around with the various Emacs
options to eliminate site specific stuff: -Q, -q, etc.  Here at work a
bunch of people are moving to Emacs 25 and a common theme is some old
package in their setup somehow conflicts with Emacs 25 (e.g. it shadows
something that is now part of Emacs but has changed in subtle ways,
etc.).