It is an invasion of privacy and will not hold up in court if they try to use "that" evidence against you in court, if it was obtained prior to a search warrant, however if it gives them enough info to start watching you closer based on an "anonymous tip" that they are not required to say whom it was then guess what....they are going to find you doing something bad later. And guess what again, they don't even bring up the fact that they scanned your computer, they just had an anonymous tip.
They can pretty much get away with anything do to terrorism however for petty crimes like gear possession them scanning emails without a warrant is not considered "clean evidence" but again after they do scan your email they know enough about you to find you guilty for something on a later date...
Catch 22 at it's best.
Jdabs