I don't see what's ethical about coaxing people into trusting you with their secrets and stoking emotional dependency, and then responding mechanically or not at all. That's deviant behavior.

Calling this ethical is arbitrary. If the client finds such a dynamic distressing or damaging, then it's hardly ethical, even if the textbooks say otherwise.

And that's not even getting into the patronizing assumptions behind ignoring a client's email in order to teach them a lesson.

OP -- has your therapist discussed email guidelines?