Centos's solution: cd /var/spool/exim/db rm -f retry retry.lockfile rm -f wait-remote_smtp wait-remote_smtp.lockfile service exim restart
If its still not working backup and remove all under /var/spool/exim/db folder
service exim stop cp -rp /var/spool/exim/db /var/spool/exim/db_back cd /var/spool/exim/db rm -fv /var/spool/exim/db/* service exim start