There are some who use Mixmaster or Cyberpunk. The NSA may know where the mail originates, but they don’t know where it is going or who reads it.
It's been years since I looked at Mixmaster stuff. There was some really interesting stuff being done with anonymous remailers 20 years aso, and I haven't really kept up. Might have to look into it again. Seems the time is ripe. Gotta figure that half the nodes are Fedgovs, as the ferals don't miss a trick, and they have an infinite amount of money.