Back to the original thread, I re-read the reply that I got from Marketo support, and realized I was referring to Email Invalid! This was my error... sorry for the thread-jack!
But... if your unsubscribe field isn't linking back to Marketo, it's likely that the two fields haven't been synced. You can either contact Marketo to remap the fields, or you can create triggered smart campaigns to resolve the discrepancies between the two systems.