If the contacts are unverified, they won't be sent to. You can also re-upload the list later with more info and it will update the contact and deduplicate the list automatically.

You can also delete all the unverified contacts too if you want.
If you click Manage Contacts on the Leads page it will give you those options.

There is also a button to unverify all leads that failed email validation. Our email validator is very strict so you may miss out on good leads but your bounce rate will be very low.