I have the feeling that this wasn't the last thing we have heard from Suzuha and really hope that I'm right.
As for how far the D-mail goes back:
If the same rules that apply to the Time-Leap also work for the D-Mail: Just to the point where the D-Mail was first activated (Taken from Kurisu's explaination)
This theory of Kurisu's would concide with the thing they probably took their idea for the D-MAIL from a mixture of D-Mail and Time-Leap Microwave :
http://www.youtube.com/watch?v=oRWwI...eature=related