I think part of the trouble with this sort of thing is the feeling or reality that the therapist changes the rules in the middle or without agreement or more gradual change etc.
For me, I find a problem with email in general is the expectation of immediacy. I, for example, tell students I will respond within 48 hours, not at all on certain days, and that there are some things I am unable to discuss via email and I will tell them directly to come see me if that is the case.