Signed messages give you a much easier way to weed out "unknown" sender accounts (=keys). Encryption with signing makes it easier to recognize legit email. You

Signed messages give you a much easier way to weed out "unknown" sender accounts (=keys). Encryption with signing makes it easier to recognize legit email. You need to go back to decentralized filtering though. Or maybe publicly verifiable signatures (=no sender anonymity) on encrypted content would be an option.