You may only do this if you have ROOT access to your Cpanel/WHM server and you must be logged on as root via SSH.  A warning, these docs are us at your own risk.  First, determine the number of frozen emails you have with this command:

exim -bp | grep -c frozen
This will produce a number of frozen emails in the exum

To remove these frozen emails use command:
exim -bpr | grep frozen | awk {'print $3'} | xargs exim -Mrm
You should see your emails delete one by one.

OK...now re-issue the command
exim -bp | grep -c frozen
You should see that the number listed is smaller.  SUCCESS.

We recommend now resetting exim to see if it restarts cleanly:
service exim restart

DONE......your mail queue should run much smoother now.