Austin Guest <guest.austin@gmail.com> writes:

> Hi there. Bit of a n00b to notmuch and having some problems using it
> alongside gnupg to decrypt emails in Emacs.
>
> I'm running Emacs 24.5.1 on Ubuntu 15.10 (x86_64) with notmuch v 0.20.2 and
> the emacs notmuch package 20160108.432 from melpa (but have had identical
> results with v 0.21 from melpa-stable).

The emacs frontend for notmuch ships with the C source. You should use
matching versions. Other versions _might_ work, but are not guaranteed
to. MELPA is kindof unuseful for things like notmuch that need C
libraries; it's easy to get badly out of sync.

On the other hand, the problem you mention sounds unrelated...

> Following the instructions on the emacstips page
> <https://notmuchmail.org/emacstips/>, I'm inserting
> `(notmuch-crypto-process-mime t)` into my `init.el` file -- after
> successfully running `(require `notmuch)`. I get the error
>
> Symbol's function definition is void: notmuch-crypto-process-mime

I think you want (setq notmuch-crypto-process-mime t); it's a variable,
not a function.