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.

How to enable IP rotation in exim.

If your server are having multiple IP and you want to send each emails in different IP address. You can follow the steps given below. This is used to maintain the ip reputation. While doing this, please make sure that this is not used for spamming.

Open you exim config,

vi  /etc/exim.conf

search for the line : interface =

replace this line with
interface = 192.168.0.1${eval:${substr{-2}{1}{$tod_zulu}}%7+1}

If your server is having ip's from 192.168.0.11 to 192.168.0.20 , you can use above line.
+1 is  is the starting ip ie  192.168.0.1'1'
%9 is the incriment value ie 11(192.168.0.11) to how much. here it is 20 (192.168.0.20)