Use a whitelist. Block any mail services that are not approved, and do that on devices as well.
It's not that hard in 2014, and makes it even more obvious when people are skirting the laws and best practices.
Hell, it's not even about skirting laws and is more about just how likely these people are to be targeted and get viruses/information compromised, etc. You'd think these people would be vaguely more interested in their own safety and self-preservation.
Yeah, I have to say as soon as I read "terrorists" I read "we're using bullshit to get what we want", because that's what I read any time I hear responses about terrorism when we're talking about both international AND domestic spying.
Nobody said it does. Crazy simply begets more crazy, especially when you are a situation where you can only see things from a specific viewpoint - everything else then is simply reaffirming your views.
Therefore, there is only one directions thing can go.
It's kinda silly. They're trying to extract blood from a turnip. They should target all internet businesses based in the US except oh wait! That would actually be even more of a pain in the ass, wouldn't it.