I just did delivery testing on my invalid emails, and discovered half were actually good - this was affecting webinar confirmations, autoresponder delivery and more. So in this case, I set up a daily recurring batch program to look for my test result field flag "Deliverable" and if that was the value, it automatically reset the email invalid field value to "false".
I think you could have a data field in salesforce like "email updated by sales" and if this value was = yes or something, a Marketo data program could reset the email invalid to false.
I'm lucky because our solutions include testing tools to identify the false undeliverables - for me it was nearly 7,000 names that had the email invalid flag set - yikes! I was happy to restore them.
Good luck!