Why are you not teaching them to fish?
So why in the name of god are you not teaching your users to fish? Collecting mis-sent emails and forwarding them on is an exercise in futility. I have valuable time to be spent trawling the web for photos of cute kittens that would be *seriously* inconvenienced if I spent my time re-addressing email all day.
Delete it and ignore it, and just start bouncing email (especially for internal only!) emails back when they screw up the address. For externals, you can bounce or drop as you see fit.