Thanks, @eviltrout and @Falco.
That’s because I was enabling let’s encrypt. rebuilding Discourse, and updating the OS. I suspect all that needed to be done was to reboot, but I thought I’d do all of that while I was there.
I keep thinking it’s my imagination, but it seems like when Discourse gets started for the first time Sidekiq doesn’t get started and emails don’t get sent. I suspect that this problem might have been solved by simply rebooting. But I was wrong. . .
In this case the problem is that Mailgun hadn’t activated the account. Since @wstudholmw couldn’t log in, he couldn’t see the [Sender] 554 Please activate your Mailgun account. Check your inbox or log in to your control panel to resend the activation email. messages in the logs. I think that it used to be the case that mailgun would deliver mail to the account holder’s address, but that seems not to be the case.