We added a custom Email Invalid field to SFDC that is synched with the Marketo Email Invalid field.
The Marketo Email Invalid field is automatically set to "True" (i.e. checked) by Marketo when an email address has a hard bounce. When you upload new leads to Marketo and some have an invalid email address, Marketo won't automatically update its Email Invalid field until you send an email to that address that results in a hard bounce. However, if you know before you upload the leads to Marketo that the email address is invalid, you can add a column with the Email Invalid heading and add the value "true" in that cell for each lead with a invalid email address. But why would you import a lead with a bad email address?