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If you're really using a webhook, this fires after the form is submitted. There is no possible way to signal this result in the form validation stage.
If you actually mean you're using a client-side integration with Informatica, it absolutely can be done (that's why Etumos Verify can kick back errors to a Marketo form after real-time validation of the recipient address). You need someone who gets the interaction between remote web services and the Marketo Forms 2.0 API. It goes without saying that there are few people with such specialization.
Then again, if all you care about is basic email syntax errors (not validation) on the client side, you don't need to call a remote service for that!