I’m still reading, but it looks like “Configure incoming email to create new topics or group messages” in particular describes what I need: to create a staged user for no-reply@around.co and give them permission to create topics by email, presumably by setting their trust level high enough or by setting the trust threshold down to 0 on the category to which they will post.
I’m still reading, but if you already know that I’m already on the wrong path or missing some key ingredient, please tell me.