This is dues to bug in the default installation of qmail in handling local mails... such as mails to root@localhost where the qmail-local binary is not set with the right owner and permissions, as such mails are not able to be written to the spool located at "/usr/local/psa/handlers/spool".
The error in "/usr/local/psa/var/log/maillog" looks like below:
qmail-local-handlers[......]: cannot create temporary file - (13) Permission denied
To resolve, change the owner and permission of /var/qmail/bin/qmail-local to reflect the same as qmail-remote.
chown mhandlers-user:popuser qmail-local
chmod g+s,g-r,o-r qmail-local