We propose a decentralized privacy-preserving approach to spam filtering. Our solution exploits robust digests to identify messages that are a slight variation of one another and a peer-to-peer architecture between mail servers to collaboratively share knowledge about spam. Categories and Subject Descriptors: C.2.0 [ComputersCommunication Networks]: General ?Security and protection; C.2.4 [Computers-Communication Networks]: Distributed Systems ?Security and protection General Terms: Security, Design