Good for you and the confronting. Way to go girl!! Because he is in the stage still of denying this I think. He left the confrontation wide open by leaving the evidence wide open for you to find. You started the process of setting some boundaries and that is very good and very brave. : )
Did you see anything between their e-mails that could give you any indication how far this has gone? I know one thing I would have asked him is how he would feel if you were calling someone hun and they were calling you baby? Would he consider that okay for you to do that? Maybe that will at least get him realizing how wrong this is. Did he say why her husband don't know if this is only a platonic relationship?
How did he act during his last affair? Did he deny ot? Did he show any remorse?
I am so sorry as I know this is hurting you. I feel for where you are right now Sue I do. Because that is where I am at. I suspect my husband is having an affair. Still looking for the concrete proof. He is being good at covering it. But when I seen him register on an adult dating site and write girls e-mails he told me he was only fiddling around! Like he didn't think it was wrong. Because he didn't mean anything by it.