Hi Stephen!
Just in the interest of Community structure, this thread should probably be in Products. (Yes, it's definitely confusing, but Marketing Central is supposed to lie outside the Marketo product support realm and be more about Marketing as a discipline. Adding further confusion is that most of the blog posts in MC are technical, even if the discussion threads aren't, since Products doesn't allow blogs for some reason. Grrr!)
Now, on to your question...
If by "seemingly legit" you mean that the addresses are @ extant domains from whom you're normally happy to accept inquiries, but the form data is spam, then you might be better off integrating a ReCAPTCHA into the form than diving into email validation. (I say this even as someone who's helped develop a validation product, as I'll note below.)
I look at validation as primarily covering real people who submit a fake address in order to obtain your content without being contacted later. A person (or their pet autoposting bot) who isn't really interested in your content, but just in attempting to advertise their own link spam or what-have-you, is a different beast.
There are a few vendors who claim to to offer real-time validation (at the time of form submit) as well as follow-up validation on the server side (for recipients that defer real-time validation attempts, as well as for batch validation of your lead database). But I haven't seen any vendors who can reliably do the real-time Marketo forms integration offered by Etumos Verify. I contributed the client-side code for that product, and as I know my way around Forms 2.0 better than just about anyone, I'm proud of what it can do. But again, it sounds like you may not need validation in your case.
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