There are many resons for this error. They may be due to dns, firewall.
Please try the fix given below.
#cd /var/spool/exim4/db
# mv * /backup
move all the files to a backup folder
#/etc/init.d/exim restart
This should fix the issue. If this doesnt fix the issue, please try by restarting the firewall. Firewall may be blocking your emails.
