It does sound like an aha moment about your mother. Not having a mother capable of emotional responding is a difficult way to grow up. But I am not sure (not like I can possibly know better than you, I'm just saying what I think) it's connected to the email.

It seems to me from what you have said that he is very nurturing and supportive, and emotionally available to you in session. If that is true, why isn't the session time enough for you, and how much nurturing really happens by email? (I'm assuming you and your mother didn't communicate by email when you were growing up. I can see wanting to repeatedly nurse at the nipple of the therapist fountain of emotionality and maybe that is what getting an email response does for you, but that seems superficial compared to what is available during an actual session.

For me, transference has been linked to a current deprivation in my own life. Looking at that, and seeing what I could do to get my needs met by my spouse or others in my social network. When I did that, my transference simmered down.