Email anti-spam (and really all anti-spam) is all heuristics now
On the Fediverse, I noted something:
Back in the days, one of the things some people said about DNS blocklists in general and sometimes Spamhaus in particular was that they were opaque, capricious, and didn't actually validate what they were putting in their blocklists, so who knows what could wind up in there for who knows what reason. Those people would take this incident as a validation of their view.
(I was going to say that this was a long standing IP address used to send Ubuntu security announcements, but it looks like we only just started to get them from this IP, although the entire IP range is owned by Canonical.)
I have bad news for such people. This is what all email anti-spam systems are doing today. There are no effective anti-spam systems that are based only on sure positive signs of spam. Everything is an opaque black box full of heuristics and uncertainty, with hopefully occasional misfires that are hopefully not too spectacular. Sometimes people hand write rules and try to assess them, sometimes people take straightforward statistical approaches (eg, Bayesian scoring), and sometimes companies go for the complicated statistics that are generally known as 'Machine Learning' or these days 'AI' (in press releases, at least).
This is not an accident and it's not because people are lazy. It's because anti-spam isn't working against a blind natural phenomenon; instead, anti-spam is engaged in an iterated game against human driven spam. If there's a sure-fire signal of spam that can be used to reject or filter email, the humans driving spam are highly incentivized to get rid of it, and only the ones who are successful at that will survive.
This is simply one of the prices that spam exacts from us. We can no longer live in a world of certainty, where we can be confident that our anti-spam systems are right about things. And sometimes we'll see things that are so obvious (to us humans, on the spot, only having to look at this one incident) that they make us have sad faces.
(There's also the related issue that no one can afford to pay enough humans enough to constantly be evaluating and updating anti-spam rules and heuristics all of the time. All effective anti-spam systems have to operate partially automatically, and sometimes that will pass things that an alert human would not have.)