If I was not on a first name bases with them in session, I personally would not. That includes if I avoid calling them anything in session. I come from a background that required last names and titles, and it has been hard to break away from. With my standby therapist I started with her tittle in first 7 emails, then switched to her first name. If these are personal poor your heart-out emails then I would use what clicks for me in my head as I'm writing it. So go for what makes you feel connected.