Could you be more specific?
Do you mean validation on the field that the entered data is an email address? Or do you mean checking that the email address is valid, form the standpoint of being able to send an email and not have it bounce?
If you mean verify, I like to use a webhook and pass the data to an api like; http://verify-email.org/register/levels.html Then based on the repsonse, flag is the email is valid or not valid with an Email Valid field. I don't believe Marketo has a built in validator that I know of.