No, that function call is incorrect. This:
isItAValidEmail($email) should be
Why do you use such an unusual file name for the ProcessEmail class? Not that its really relevant, but would cause huge grief should you change to using autoloaders.
Also, "client" side emailing is a simple mailto: call. This you have is server side; the AJAX does not make it client side it simply makes it's request transparent.
header('HTTP/1.1 420 Enhance Your Calm');