Check out the following reports for more information
This blacklist lists all IP addresses that are unlikely to be used by a legitimate mail server. Legitimate mail servers should use a static IP address with a non-generic PTR record and that reflect the host and domain name of the mail server and ideally should match the forward lookup for the same name (FCrDNS).
It is designed to catch botnet traffic, compromised hosts, hijacked IP space and compute/VPS hosts.
This list is generated by running a reverse DNS lookup for every IPv4 IP address and lists IPs with:
Any host containing smtp, mail, mx, mta is automatically excluded.
Please do not request a delist from this zone unless you are running an email server on this address. Being listed in this zone will not affect your ability to send email unless there is a mail server running on it.