Plesk 9.2.x - Postfix SMTP authentication error

postfix/smtpd[24579]: sql_sqlite3 plugin database open error /plesk/passwd.db: unable to open database file
postfix/smtpd[24579]: sql plugin could not connect to host localhost
postfix/smtpd[24579]: sql plugin couldn't connect to any host

Because of a bug or configuration error, the postfix is not chrooted, but it search to "/plesk/passwd.db" file.
You can symlink the directory like this:
ln -s "/var/spool/postfix/plesk" "/plesk"

Another problem, is that the passwd.db could be unreadable by postfix program, because it runs as the wrong user. You can do a quick workaround, by allowing r/w access for each users, but this is dangerous because the password file will be available for each logged in user on the server:
chmod a+r+w /var/spool/postfix/plesk /var/spool/postfix/plesk/passwd.db