If you use something like rspamd, then doing ML is not terribly hard. Then it becomes a problem of providing a proper teaching set. Combining RBL's (be it URI,

If you use something like rspamd, then doing ML is not terribly hard. Then it becomes a problem of providing a proper teaching set. Combining RBL's (be it URI, IP or domain WL's or BL's) will help with that. In the end it's still a hard problem, so takes a bunch of work.