Greetings! If I save an email from john@example.com on my VPS and then pipe it into `sendmail peter@example.net` Peter will receive the original message as if John had sent it. I'm even more surprised that you can do this between Google accounts (Gmail and Google for domains). Why is this practice allowed? Clearly Google knows the message is fake in the sense that John didn't send it to Peter. I haven't tried to abuse this feature, and that's probably why it works under my tests, but I wonder why Google lets these messages through at all. Any ideas? What would be a legitimate use for this?