I couldn't understand why so much spam got through the filters.
Yesterday and today I checked 667 addresses that have tried to send me spam in the past days. 661 of the 667 are listed by the Spamhaus Zen list right now, but Google's public DNS resolvers report that only 33-74 are listed, so Spamhaus is effectively neutered.
Google offers two IPv4 and two IPv6 addresses. All four report similar numbers, and the results have been largely stable over the past 24 hours.
A smaller RBL operator does not suffer, so I expect it's some sort of rate limiting by Google. DoS mitigation gone wrong.