You have a fine line to walk right now. By deleting her e-mail, you over reacted. simply changing the password for a week would have been sufficient.
Here's the thing, from a mom of 8 kids, 4 of them adults. Many, many schools actually require students to have their own e-mail and will even walk a student through setting one up. Many teachers use this as a way to communicate with their students.
There is also a difference between being proactive with your child and a computer, and protective. Sure you want to make sure they are safe but the best way to do that is to be proactive. Talking about the dangers, reminding them to not give out ANY personal info. Only talking with people/family they know. Then taking a step back, letting them find their way, and stepping in when they need is there.
My children, including my young ones one do not need to ask for permission to go on the computer, or their tablet's. I do have some parental controls, but probably a little looser then some. I periodically check the history, have yet to find anything of concern. They know that I can and will access their accounts when I feel like it, which I don't normally feel like.
IMO, the younger a child is, when they learn how to navigate the internet. The better off a parent is in the future. By the time they are teens they are capable of doing the right thing, because it is so ingrained in them it's second nature.
Remember, every time she steps out that door. There is a chance to access the internet. The question becomes...do you want to know what she is doing or will she just hide it from you...?