We have been sending mail campaigns successfully. Then, all of the sudden, for no apparent reason, we received hard bounced errors for 3 contacts and from then on, we have not been able to send out emails to them.





We thought we were blocked by their SMTP servers but we just found out that there were black listed at GS end. When mail hard bounces, the admin has to run the email validator, type in each email address to correct it.





For all of them, we confirmed they were blacklisted for the following reason: 554 Blocked





and we are referred to a link in ipcheck.proofpoint.com where we can read the following with Q&As





About Proofpoint Dynamic Reputation (PDRTM) and steps you can take if your IP is being delayed or blockedProofpoint Dynamic Reputation leverages Proofpoint's world class, machine-learning driven content classification system to determine which IPs may be compromised to send spam (i.e. as part of a botnet). We understand that different companies and different people have varying ideas of what 'spam' is. We take a stance that it is better not to block some messages (spam) than delay or block legitimate email that our customers are expecting - this policy is an extension from our content classification system. The goal with IP reputation is to delay or block IPs that have been identified as being part of a botnet or under the control of spammers.





Why, at the sending end, are is the contact blacklisted?





How can we prevent this rather than correct it?