I work for an ISP that purchased an IPv4 /21 from an IP broker.
About a year ago we purchased the /21. At the time we were told that the CIDR was “clean” and it seemed to pass all blacklist checks. We have recently had many downstream business customers complaining of their emails not going though, or getting bounce backs. It seems to lead to a larger CIDR that our /21 is within being on the Symantec bad rep list.
When doing a look up at https://ipremoval.sms.symantec.com/ipr/lookup I have found that IPs within that /21 that have never been used within our network are on the list. It appears that IPs that are in the /19 or maybe even greater are all on the bad reputation list. What steps do I need to take to prove ownership of the /21 block, and that it is “clean” so I can get it removed from the list and resolve the issue for my downstream customers.