You can leave the door ajar...being civil/friendly doesn't mean that you have to respond to his emails/texts/calls right away. If you do not feel that you can move on w/the constant contact, cut it back a notch.
He needs to learn and understand that what he has done has created a mess and a divorce is in the making. You, on the other hand, have to set some boundaries about the contact and only you can do that.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
