Reply by email in a self hosted installation

I'm having some trouble getting the reply-by-email feature going in a self-hosted installation. I can send the email fine, it gets received by the Mailin docker image, and it apparently is processed by the app, but the activity doesn't get added to the discussion.

I've included the logs generated when trying to comment on a thread by responding to the notification email. (Note that I've replaced references to my domain with loomio.example.com.)

The logs indicate that there's a problem with the spam checker, but my interpretation is that all incoming mail is therefore getting processed.

Beyond the spam filter it looks to me like there are no problems surfaced in the logs. Can anyone spot something I've missed? Or are there any other tips you have for making this work?

Ok, posting the log excerpt here was ugly. Here's a gist:

https://gist.github.com/cheesington/101847f4e589160e9e9faf6364d574b5