Tell him since he is friends with this woman, and you know all his other friends, that he should invite her over to dinner, so you can get to know her also.
Suggest fixing her up with one of his single friends so you can all "double date".
If it is all innocent, then neither of these suggestions should bother him.
What is the quality of their emails? Is it just chit chat about jobs and long lost mutual friends and daily life? Or is it more intimate - thoughts, feelings, longings? Some of my male friends from school and what not, are married, but we chat occasionally on FB - BUT their wives know me also, as we have all known each other for years.
You need to sit down, calmly, and make clear to him that you are uncomfortable with his talking to a women that he used to like. Your feelings should come first with him.
I don't think a few emails implies cheating...but, you need to make your feelings very clear to him so he knows where you stand on this matter.
Good Luck and God Bless