If you receive complaints from people sending mail to you with the following error "retry time not reached for any host after a long failure period", it is likely due to an error with the server sending mail at their end. The Exim Retry Database has likely become corrupt and would need to be fixed before they could retry sending mail to your email address/domain.
For reference, the procedure for fixing this (if you want to look knowledgeable to their systems administrator) is as follows:
If using cPanel with CentOS, as root user run the following:
rm -f retry retry.lockfile
rm -f wait-remote_smtp wait-remote_smtp.lockfile
service exim restart