Never forward email to GMail. Ever. Google ’causes’ backscatter.

Google is doing exactly what it should do, and what is standard industry practice… Silently dropping mail is generally considered to be bad, and I don’t know any major email service providers that do it. Hotmail sometimes does it, but usually doesn’t.

Rejecting with a 5xx code is the correct response when you don’t want to accept a message. It is your server that is generating the backscatter by accepting the message and then ultimately bouncing it.

Email forwarding is just generally not very good. If you can avoid it, do so. Perhaps you could use Google Apps for your Domain?