cPanel install, sending via phplist DKIM fails


I have the latest phplist 3.3.3 installed on a WHM/cPanel box (latest version). I have setup phplist to send via SMTP but “some” are failing DKIM check with the error “body has been altered”. Everything is hosted on the same machine (no separate mail server)

This has been working without an issue but for some reason this has now started and I cannot work out the solution.

Any assistance would be great.