I believe the suggestion is to do something about the account on the other service that has been pointed to your address, e.g., if United Airlines is sending you mail about flights you never booked, perform a password recovery on the UA account thereby locking out the person that entered the wrong email address. I'm not sure I agree. But then it doesn't often happen to me.
That could be inferred as unauthorized computer systems access under various laws, so I don't personally, but it's definitely something you can try to do in varying degrees.
