One more piece of the puzzle: the explanatory comment text in
/etc/dovecot/conf.d/auth-system.conf.ext:
# LDA and LMTP needs to look up users only from the userdb. This of course
# doesn't work with static userdb because there is no list of users.
# Normally static userdb handles this by d
I'm still very confused, but I have finally gotten it to recognize
all users. What changed from my previous message is that I deleted
the static userdb, and added the system user to the
/etc/dovecot/users file (but not to the passdb /etc/dovecot/passwd).
Oddly, I had to set uid:gid and homedir
