[Dovecot] using dovecot for postfix sasl

[h at hpjt.net]

I have followed the Dovecot/Postfix/SASL howto, and modified the main.cf
and dovecot.conf files, yet when trying to authenticate during a postfix
SMTP session the log reports:
"postfix/smtpd....warning: SASL authentication failure: Could not open db"

Dovecot itself is authenticating IMAP logins correctly, and it is
creating the auth file in the location specified under
/var/spool/postfix/private.

Why is postfix not able to get to the Dovecot auth mechanism?

Thanks.
Hugh