I believe in always giving responses within certain thresholds, so that any breaks in mail can be properly diagnosed and resolved. Preventing directory harvesting can be done by setting limitations on how many invalid recipients you're willing to accept within a given time scale from a single sending IP. I normally say 5 over a period of 60 seconds. If a user on someone else's mail systems has that bad a spelling impediment, then tough :)

Dropping silently unless you're sure that you don't want the mail and don't care about the sender is not cool. A simple DSN with enhanced status codes goes a long way to making a humble mail admin's job that much easier. It gets tricky when you're trying to comply fully with the RFC's but also secure your mail systems against abuse. Not that I'm complaining. It keeps me employed :)