It is applicable for qmail + Plesk

Attention!

All current messages will be removed from the queue in this case and cannot be restored.

1. Stop Qmail and xinetd.

/etc/init.d/qmail stop
/etc/init.d/xinetd stop

2. Move current queue to another location.

mv /var/qmail/queue /var/qmail/queue_old

3  Reinstall ‘psa-qmail‘ RPM to recreate qmail queue structure with the command like:

rpm -Uvh --force psa-qmail (Redhat, Fedora, Cent oS )
dpkg -i psa-qmail (for Debian or ubuntu)
4. Start Qmail and xinetd:
/etc/init.d/qmail start
/etc/init.d/xinetd start
Qmail Queue Check ..

/var/qmail/bin/qmail-qstat
All Qmail Configuration Check up :

/var/qmail/bin/qmail-showctl
Use the qmail-qread utility to read the messages headers:# /var/qmail/bin/qmail-qread