JdeBP> Authorization to receive mail can be handled by the normal
JdeBP> user database mechanisms.

DS> Unless you're dealing with users with shell accounts, which
DS> can be handled using qmail-users.

I classify "qmail-users" as one of the "normal user database mechanisms".

JdeBP> Authorization to send mail (via SMTP Relay) can be handled
JdeBP> using the ${RELAYCLIENT} mechanism, or (if that is
JdeBP> insufficient because the SMTP Relay clients are to be
JdeBP> outside of one's own networks) by the smtp-auth patch
JdeBP> (as long as it is configured correctly (-:).

DS> That won't work for users with shell accounts. [...]

He did explicitly give the proviso:

D> Please note that qmail would only be used as a
D> SMTP MTA and nothing more (no local mailboxes).