To find out the Email deliverability rates, go to the smart campaign you used to send the email and then click the 'Email' tab. You will be able to see delivery % as well as soft & hard bounces.
When an email address returns a hard bounce, Marketo marks field 'Email Invalid' to "True" and stops attempting to send emails to that address.
Here's an articke that explains in more detail: https://community.marketo.com/MarketoArticle?id=kA050000000KzZyCAK
I sent out an email to the whole list from the Lead Database tab. And I don't see Email as a tab there... See screenshot.