I would just ask her how she can be reached out of session if needed. I think most Ts are available over email for at least scheduling issues but there are always exceptions. You have every right to ask either way - it is not intrusive at al if that's a concern. If she has an email you can ask if she follows any sort of protocol. Generally you can email a T anything you want - the question is what kind of response you'll get. Some Ts don't respond but rather print the message and put it in your file to discuss in the next session. Some email a short reply saying that they'd rather discuss in session. Some will reply consistently and at great length.

My T and pdoc are in the same practice and respond very differently. T actually doesn't respond at all (she doesn't seem to check them or she forgets) and my pdoc usually responds but always via a phone call. Confidentiality can be an issue also, so if your T is with an agency or group practice it's likely emails are filtered through their secretarial staff before they even see it. That is a question to ask as well if she doesn't tell you (which she should) and may determine what you'd use email for.