I've also had some clients get upset when I did not reply right away. I've actually processed this issue with one person in therapy as an example of delaying gratification. I agree that it won't work for some.
I once heard Richard Kluft say that he tells his patients "I'm on call, not on tap." At the time I thought that was an insensitive statement. I've warmed to it. When you compare email to a phone call it is an example of delaying gratification. It can impede treatment if it begins to take the place of sessions, I think.
Leonard Holmes