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 addres

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.