Funnily enough, I already discussed RBL technology a long time ago, and there's even an issue on it.
Making an RBL is fairly easy. I prototyped one in an afternoon using rbldns. The harder part is determining what hosts (or IPs) go into it, and continual maintenance.
I've also written up (somewhere) a set of RBL status IPs (ala the 127.0.0 block used by traditional RBLs) that reflect more fediverse-type issues.
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