Why would a system-level IP block affect only some addresses at a domain?

I have 3 contacts at an account that were sent the first two emails of a Welcome series. Two of the three contacts hard bounced for the second email, and one contact has successfully received the first two and a third email. When I look up the bounce reason, it states:



421 4.7.1: Data command rejected: IP blocked for behavior.



Wouldn't the third contact also be blocked at the recipient domain if it is blocking the IP address (all contacts were emailed in the same batch)? Should I remove the blacklist for the two contacts or is it a valid suppression? Is there a report I can pull to provide all bounces with their reasons YTD?



How strictly does Gainsight monitor its customers' sending habits and their impacts to overall deliverability? I feel like I see more 'global suppression' type bounces than I've seen at other ESPs, and those were prospect email lists which I'd argue are higher risk than what should be primarily customer contacts for any of Gainsight's customers. A support ticket indicated it should be 'rare' that customers would share the same contacts and thus impacts from the global suppression should be minimal, however with Enterprise-level customers I'd argue the opposite and we are in-fact being affected by the global suppression.