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
[To see links please register here]
function.
Hidden Content
You must
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or
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to view this content.
In case you are looking for the second thing take a look at the
[To see links please register here]
function of the PayPal API.