This blacklist generally adds /24 or /16 catch all entries when repeated spam comes from a /24 or /16 and they don't foresee any legitimate email coming from these hosts.
Schulte Reports Subnets
Subnet-based Blacklists are used to reject email from entire ranges of IP Addresses, i.e. providers that are hosting companies sending spam, as well as single IP Addresses that may fall in that range of IP Address.
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