Safari makes it too easy- Command-Z works fine.
Why is there so much outrage at this? Jailbreaking works by first finding bugs and vulnerabilities and then exploiting them. Yes, Apple is preventing jailbreaking, but they're also securing their OS.
Domain Keys authenticates that the message was generated by a server with access to the DK private key. Forwarding the message does not affect the originator of the message, so the Domain Key authentication still checks out.
SPF and DKs solve similar issues, but in a much different manner.