Obviously, if the serve is really close and you are returning and calling the ball out at the same time, there is nothing you can do about that. But if the serve is OBVIOUSLY out, then you choose what to do…
• Hit it back across the net? Not sure there is anything in the rule book, but common courtesy says that you do not do this. One friend reported playing a match where his opponent constantly returned every missed serve… and said it was his right to do that.
• Hit it into the net? This is what I normally do; but another friend suggests that it can end up distracting YOU more on your next return than your opponent.
• Let it go behind you? Many have suggested this is “the right answer,” but my concern is that all too frequently the ball hits the fence and rolls back onto the court, stopping play while you have to chase it down.
Is there a right answer?