We are having this issue happen as well. Emails are being marked as a bounce with a 550 reason and not being delivered however our Sales team is saying they are able to deliver via outlook. Do you know why this might be?
No they aren't suspended all I see in the activity was bounced then marked invalid at a later date because of a campaign we have running. The record is not marked suspended. I'm just wondering why they were only attempted delivery once and not again after the bounce.