I agree that there are pros and cons of emailing your T. I've always been able to express myself better and more freely in writing than in speaking. I jumped at the chance to be able to email my T because my current T is the first T to allow it. (Years ago, we didn't even have email of course)
I like the chance to "get it all out", especially right after my session when all the feelings hit me. Some say it's better to wait until the next session, but I can't sit for a week with all of my feelings. So, in that sense email works for me.

What doesn't work is expecting a certain kind of response from my T. That's where the disappointment comes in with email, I think. Then there is the urge to email again, and have T email again, and it can spiral, like it did with me once.

Another reason I like emails more than phone calls is that I have a record of what I wrote, and if T responds, what she wrote. I forget a lot, and it helps me to reread what I emailed to my T right after a session. When I want to connect with my T, I reread some of the many emails I printed. They make me feel better and help me remember how much she cares about me and how she is helping me.

I agree that emailing depends on the T and the client; it's an individual decision on both parts, and needs to be thought about carefully, and rules may have to be changed if it's more harmful than helpful.