I’ve updated our error message thanks to the tips in @simon’s message above and suspect that this will be sufficient to address the confusion that some in our community have felt. If there were a way to customize the “From:” address to something more like noreply@discoursemail.com that would be icing on the cake, but for now I think we’re in a much better place.
Thanks again for all the help here,
-Brad
PS — As long as we’re bagging on Thunderbird in this thread, I’ll mention that it’s still perplexing to our Thunderbird users that URLs from their HTML-formatted messages seem to get dropped on the floor by Discourse’s incoming mail processing filter, where it remains unclear to me whether this indicates a Thunderbird bug or a Discourse bug.