PS:
I also have this line in /var/log/maillog
May 16 20:56:23 blue vpopmail[22654]: vchkpw-smtp: vpopmail user not found nick@:127.0.0.1
..which implies that perhaps it's only looking for vpopmail users rather than /etc/passwd users.
I would like it to work for /etc/passwd users only, or both - whichever is easier.
Thanks!