Spool file is locked (another process is handling this message) : Unable to send emails ,exim

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.