This is bad news, because personalisation was a big advantage of spam filters. Everyone's spam filter is tuned differently from others', so spammers had a hard

This is bad news, because personalisation was a big advantage of spam filters. Everyone's spam filter is tuned differently from others', so spammers had a hard time beating this with automated messages. About the best they could do was adding random keywords in hopes of triggering someone's positive "not spam" trigger. Now spammers gain personalisation at scale, so this advantage is at risk.