cPanel Email Error: "retry time not reached for any host after a long failure period"

This can be caused by a corrupt database file.

Fix it by running the following SSH commands as root:

cd /var/spool/exim/db
rm -f retry retry.lockfile
rm -f wait-remote_smtp wait-remote_smtp.lockfile
service exim restart

or

/usr/sbin/exim_tidydb -t 1d /var/spool/exim retry > /dev/null
/usr/sbin/exim_tidydb -t 1d /var/spool/exim reject > /dev/null
/usr/sbin/exim_tidydb -t 1d /var/spool/exim wait-remote_smtp > /dev/null
/scripts/courierup -- force
/scripts/eximup --force