How to add criteria for 'When no email address' in WorkFlows

Hi all,

For when a Lead is created I want to create a WorkFlow to check for if an email address is present. I dont think email address is part of the fields I can select so is there a way I can achieve this?

In the screenshot below there is no 'Email Address' field I can pick. We have a custom field I believe or out of the box that is 'Email InValid' which is just a checkbox. This filed 'Email InValid' wont work for us as its only reliable or correct when an email address is present.