HI Bin,
Yes, I've seen some companies send an email to verify their email address by requesting then to verify a link in the email. It's a great way to get better quality data by doing this. You can do this by creating a smart campaign that triggers for any form fill out and send an email to verify email address. Then create another smart campaign that triggers off of the clicks link in email and either add them to a static list or mark a custom field that this person has verified email address.
Marketo also has information if the email is invalid and the email invalid cause. You can find that information under the lead info & you can also pull a smart list.
Hope this helps.