Husbands ex girlfriend contacts him after 12 years via email found a hand written note that he was answering.

Husbands ex girlfriend from 12 years ago sends him an email on his birthday in November 2008, she got his address from husbands friend who sends jokes in via email and added hers, the exes to the list. Then supposedly sends him another email 5 months later to say hello and how are you doing? He never told me about the 1st time she contacted me as he knew I would be mad about it. I find out that he is answering her when I find a written note on the desk that he had written so that he would be able to type the answer back to her, she was supposedly asking him for advice about a couple that they used to go our with and was getting divorce. I wake up in the morning and find this note. As my husband goes to bed after me and is on the computer and now I know why? I fall apart when I find this note as now I know that something is going on behind my back...when I confront him he is oh my god like what did I do forgot to throw the piece of paper away and tries to explain it was only twice and nothing is going on , but I am so badly hurt that he never told me about this opened the door to her and continued to email her back instead of asking her to leave him alone. I want you to know that we are in our 60's and we were married when we were very young had 2 children, divorced remarried other people divorced and after 25 years have remarried. All I want is peace at this point in my life , he can't see why I am so upset.......I don't know if it is continuing and I don't know how to find out if it is. He promised that he toldher to stop and I emailed her myself to let her know that I knew she had been in contact with him and I wanted her to stop and leave us alone. What do I do as he is now wanting to talk about it, gets very upset with me when he sees that I am in the mode where I can't let the thought of it go. What do I do to find out if it is still going on?