I realize this posting was several years ago, but my comment is as relevant today as it was back then:

Due to HIPAA laws, including updated laws in 2013, it is common practice for therapists to simply reply to emails by stating that they have received and read them.

Emails are NOT considered 100% confidential under HIPAA laws because they go through the internet and are stored on servers.

This is not to say that your therapists are not going to keep your emails confidential, I'm not saying that at all. It just has to do with third party gray areas like gmail, SBCGlobal, etc.

I would consider not putting your full name on the emails, just for security reasons.
I worked in I.T. and I know ALL the private emails were read (!) in both esteemed companies I worked for, so be sure to only use your PRIVATE email.

Just try to remember that therapists have strict ethical and confidentiality laws they must abide by.
Chances are, if your therapist has encrypted email, he/she may respond in more detail. Others are most likely just following the HIPAA confidentiality laws. In other words, it's nothing personal against you. Not at all.

And by all means, talk about their lack of lengthy responses during session.