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.
That isn't bad. The only obvious flaws are running out of unique addresses or, if addresses are easily guessable, having spammers iterate over your free addresses inserting themselves as the "first sender."
A system where that person is added as the sender to the system BEFORE they send an email for the first time would solve this, however, this is equivalent to white-listing in current email solutions.
Any non-obvious flaws?