Internet Storm Center

Nick submitted a nice piece of malware we are currently looking at. The malware itself includes a nice rootkit that doesn't appear to be detected with common rootkit tools. However, in my opinion, the delivery method was noteworthy. It used a spoofed internal sender. This made me wonder what's the best way to block e-mail that comes from outside mail servers, but claims an internal "From:" header.

My first guess would be that this is a great reason to enable SPF. Will this work?  I am assuming that it is a standard policy to require employees to use a VPN connection or "something like that" to send e-mail using an internal mail server.

Any other ideas?