I have exchange 2010. If I go to https://emkei.cz/ and send an email by entering an email address that exists on my exchange server in the "from" box and send it to my gmail address, My exchange server goes ahead and sends the email. Gmail blocks it because its spoofed. Here is the deliverable message I got in my inbox from Gmail.
If I use this spoofing website to send an internal email, lets say from my co-worker to myself it arrives safely in my inbox. I have looked through my send and receive connectors and cannot find a way to stop this.