We (Etumos) built one of these! Specifically meant to do this exact thing with Marketo Forms 2.0. I call it one of our microproducts.
Basically, it's some JS on a landing page with a form that watches for the email address input. Upon submit, it does a real-time MX lookup on the email address to tell if it's a valid email or not. If it isn't a valid email, pops up an error to the user in the same format of errors as you already styled with CSS. Then it's also available as a webhook in Marketo with some more features for the response, such as telling you if it's a free email account domain (e.g., @hotmail, @gmail, etc.) or otherwise.
You can play with demo version here: Demo: Real-Time Email Verification in Marketo
We usually roll it out in a more comprehensive package, but we're starting to experiment with selling it as a stand-alone. Feature requests welcome. : )
Edward Unthank | Founder, Etumos