Be sure you clear it first with your T.
Understand exactly what the ground rules are - such as how often can you send email, and when should you expect a reply - if at all.

This was one of the first things I did with my T as he told me I could email him as I worked through my trauma healing. Email is a tool my T uses for very specific reasons in therapy. In truama healing, the flashbacks happen at all times, and it can create an interal crisis when they hit. So emailing T at 1AM is better for the client (you don't feel like you are bothering T with the stuff - which adds to guilt) and T isn't being disturbed from sleep or other sessions all the time when there is nothing they can really say or do to help because the bulk work of the trauma healing is to actually get IT all out into the open and process the emotions.

Waiting around on T to email you back if the rules are not clear can be retraumatizing. That is why it is important for a client to understand what will happen when an email is sent.

My T told me he may not even be able to read the emails I send him all the way through, and that kinda stung my heart. But he did want me to have that outlet. So even knowing that, I was able to use the tool for healing. We may not like the rules our T has in place for things like email, but it is much better to understand the rules up front. So ask.