Today after the latest cpanel update I was unable to send mail. I spent several hours on this and finally found the root cause.

Appearently, when cpanel did the update the /etc/exim.pl file was deleted and not replaced with a new file.

I simply copied /etc/exim.pl from another cpanel server and was able to send mail instantly. This might help you and might not but it was the solution that did it for me!