If you're getting something like this in your exim logs (exim_rejectlog, in my case), I believe this is caused by a file permission issue caused by a cPanel update.
The permissions on /etc/exim.pl are set to ----------, not -rw-r--r-- as it should be.
I updated the permissions
chmod 644 /etc/exim.pl
service exim restart
PHP mail is now sending emails
The permissions on /etc/exim.pl are set to ----------, not -rw-r--r-- as it should be.
I updated the permissions
chmod 644 /etc/exim.pl
service exim restart
PHP mail is now sending emails