Single sender email addresses -- The idea is that a subscriber to the service gives out a separate email address to each friend or associate that he's willing to hear from (maybe from serialized business cards or whatever). The mail server accepts and records the first sender to any previously unused address, and subsequently performs a policy dependent action (e.g., blocking) when mail to that address is received from any other sender. Return addresses on outoging mail also have to be handled consistently. Not sure if there's a market but I know I'd use it myself.

Any non-obvious flaws?