Great; that worked. Next question is how come?

I understand that starting authdaemond would fix the authentication problem, but
none of the other daemons are running at this point, so how does mail make the delivery? What would happen if the delivery
needed to be queued, because it was for delivery to a (temporarily) unreachable host?

This isn't just an idle query; I want to replace sendmail on my bastion host. At
the moment, smtpd is listening on port 25, and hands off all messages to sendmail for forwarding to either internal or external
hosts - there are no local mail accounts on this machine.

I am perfectly happy with smtpd's blocking/security features, but would like to
replace sendmail with something better - by which I mean courier.

Ideally, I would like esmtpd to listen on port 25 on the internal and loopback
interfaces (so my real mail server can use the courier to courier delivery optimisations), with smtpd listening on the external
one. The courier version of sendmail will need to be able to forward mails coming in from outside via smtpd. Easy so far. But what
happens if the internal mail server is temporarily down? At the moment, I run sendmail as a non-listening daemon as well, so it can
process its queue from time to time. Which components of courier do I need to install and run to achieve an equivalent
effect? Is there any documentation that explains exactly what each module does, and how and when it interracts with the others - man
sendmail isn't very helpful on this, for example; it states that it does some things itself and relies on other modules for other
functionality...