I don't know that this is purposeful in all cases. I've had issues on a mail host where sending an email to a different box on the same domain would reject the

I don't know that this is purposeful in all cases. I've had issues on a mail host where sending an email to a different box on the same domain would reject the message as spam, refusing to even deliver it. With respect to google, however, the fact that they offer a service of their own that conveniently can avoid this issue is more than a little shady.