Here's an idea: public mail pools. The entire email message is encrypted and randomly routed to one of many addresses. Each customer downloads all the message

Here's an idea: public mail pools. The entire email message is encrypted and randomly routed to one of many addresses. Each customer downloads all the messages from all the addresses in the pool. You try to decrypt each message, and the ones that decrypt are (obviously) yours. The rest belong to someone else. To make life tougher on spies, add fake email messages to the pool regularly.