DomainKeys doesn't work if there is any uppercase in the From address, eg. john@Example.com
A lot of people use uppercase in the From address, to increase readability, branding, etc. I have not found any specification that says DomainKeys should be case-sensitive, and my research on RFC specs indicates that hostnames should always be treated as case-insensitive.
Therefore I believe this is a bug in the implementation by cPanel which I hope will be addressed.
Also until this is fixed, I hope that this message is helpful in hopefully saving people from the hours of frustration which I experienced while trying to figure out why my emails weren't getting signed