Seems to me the main thing is to establish expectations and ground rules around email. Once that is set, the T needs to be consistent and predictable.

Regardless, allowing a client's email to go unanswered for days is not ok in my book. It's a sure fire abandonment trigger for certain people. The silence, for me, also carried implicit messages of shaming or admonishing. I also don't like the way clients are made to feel guilty for this, and the way some Ts act like even a short reply is a terrible hardship for them.