If you are receiving the following error in exim_mainlog file then refer to the following solution.
Exim : retry time not reached for any host after a long failure period
The issue is because of the corrupted exim db files.
Go to /var/spool/exim/db and delete files: retry , retry.lockfile , wait-remote_smtp, wait-remote_smtp.lockfile
#rm retry retry.lockfile wait-remote_smtp wait-remote_smtp.lockfile
After that restart the exim service.