Does the system, as it is supposed to work store any emails that aren't applicable to a warrent? No, unless they've changed it from the last time I knew something.

Can it be used to store all that email.  Well, yes.  Then again, you could say that they could just tap every phone too, so there is always risk in monitoring devices, no matter what they are.

However, since the proper use of it doesn't keep it, thats why its legal.