Qmail not sending mail: qmail-queue[26720]: cannot reinject message to mail system
Stop qmail:
service qmail stop
service courier-imap stop
service xinetd stop
Backup the bin and qmal queue directories:
mv /var/qmail/bin /var/qmail/bin.old
mv /var/qmail/queue /var/qmail/queue.old
Now refresh this installation. To do this, you will need a fresh copy of the psa-qmail rpm for your distribution. You can either download a tarball of all the rpms from the plesk website.
So then, refresh by running:
rpm -ivh --force psa-qmail*
Try again, and you should have overcome your injection problems.
# service courier-imap start
# service xinetd start