Thanks @maiki. Tested again. Sent in an email. As an admin, I hit “Reply” in the corresponding Discourse category and sent the reply on its way. But to where exactly I have no idea. There is no trace of anything happening that I can find anywhere other than the record of the reply in the Discourse category itself. Nothing in the Discourse email logs. Nothing in SendGrid logs. And definitely nothing arriving back in the originating mailbox.