There are a couple of ways that you can accomplish this. If the flow included a "Send Email" flow step, you can create a smart campaign with "Member of Smart Campaign" AND "Not was sent email" filters.
Since the leads that are blocked from mailing would not have been sent an email, only those will qualify for the criteria above.
If no email was sent out, then leads that were blocked from mailing were processed along with other leads anyway so it might not be too helpful to know who these people are.
You can try
Was Sent Email X on Y Date
Email Bounced IS X
or just Email Bounced Is X on Y Date(s)
that will tell you who is likely rejected after each email. BUT, Marketo usual suspends those addresses for 24 hours, it does not always invalidate. You have to setup a system to do that yourself.
Thank you, Raul! This is exactly what I needed to know. I need to pull a list so I can tell the Customer Success team which contacts didn't receive the newsletter.
Rachel, this article will help you setting up a smart list or a segment to identify leads that are blocked from mailing: Why are these leads blocked from mailings?