It's easy to set up secure communications within a small, trusted group. So this won't affect any real terrorists that are organized enough to be a real threat. They just install PGP (for example), just as anyone else can. And since the security is end-to-end, it's secure no matter what mail system it passes through. And no matter what laws anyone passes, math still works, so end-to-end encryption is secure from anyone attacking the security. And it's open source, so they can't sneak in corruptions to subvert security. Math doesn't care about politics - if the attackers are your government, or foreign attackers, it's all the same math that protects your communications.
What it will do, though, is let them collect tons of data from from people who aren't serious terrorists. Think of the fun the can have with that!
The real answer to terrorism isn't increased surveillance, or the "magic pixie dust" of data mining, it's real police work. That's what's stopped ever terrorist attack (that's been stopped) so far. If they cared about security, instead of surveillance or big equipment contracts, they'd focus on the stuff that works. Hire lots of smart people, train them and equip them, and pay them well, to do the hard work. The rest, attempting to outlaw encryption, scanning people's shoes, etc., is all a stupid waste of time and money, degrading our society's freedom (i.e. doing what the terrorists want) while achieving nothing of value.