We use several third-party blocklists, and one of our own, to determine whether or not to accept a connection to our inbound mail servers. Those blocklists are:
If a connecting IP address is on any of these blocklists, the connection will be rejected with a message that includes the rejected IP address and a URL pointing to the blocklist in question; the error message will look like this:
554 5.7.1 - ERROR: Mail refused - <IP ADDRESS GOES HERE> - See URL OF BLOCKLIST GOES HERE
If the reason for the rejection is due to any of the third-party lists mentioned above, the party responsible for the IP address should contact the blocklist maintainer, not us, as we do not control the content of those lists.