Configuring sonicwall to exclude all IP's from accessing Port 25 except google

I have a TZ 210 and I have google apps purely for filtering to our exchange 2007 box. My problem is that I cannot seem to figure out how to configure the sonicwall to block all port 25 communication from IP's except Google Apps. Typically this would be a simple IP range on the firewall but according to google they require you create SPF records. There's no option to have it block based on that. I contacted sonicwall and they just seemed more confused than I was about it all. Here's the link I was wondering if anyone could help me get this set up as we're getting HAMMERED with spam.

https://support.google.com/a/answer/60764?hl=en