[chunky]

What exactly do you mean by valid email? Do you want to check if the entered string is a valid email (like containing an @ followed by a valid domain) or do you want to check wheather the entered email is verified by paypal?
You can check if it's a valid email by using the filter_var function.

Code:

if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
   // Valid email
}

In case you are looking for the second thing take a look at the GetVerifiedStatus function of the PayPal API.