You have to tell your husband how this is bothering you and that you can not get over this, it is never good for a married man to be in contact with an ex especially when he tells you he has thought about her often and was curious those are two very big warning signs that you need to tell him to stop corresponding with this woman even though she lives in another state, just the fact that she is divorced and single should be another warning sign and just because your husband told her that he is happily married and doesn't plan on leaving you doesn't not mean that she won't try to get him unfortunately nowadays some people's morals aren't as great as others and they are selfish and don't care who they hurt including the children because they are thinking of their own selfish yearnings.
Ask your husband if the tables were turned if he would care if you contacted and old flame and had so much contact with them, how would he feel? When your husband married you he was to give up his old ways and the two of you were to become one. If she is someone that you wouldn't or don't talk to in the same manner then your husband shouldn't be either. If you want your marriage to stay the way it is and not change I would suggest you tell your husband you don't feel comfortable about him talking to this woman and that it can only lead to problems in your marriage and if he does love you as much as you say he does he will do what is best for his family.
If you don't want to lose your family you have to put your foot down where this woman is concerned she is divorced meaning she is single now and your husband isn't and since you don't know the extent of the conversation I wouldn't let it go because you don't know what they talk about and though it may be innocent on your husband part you just don't know if it is innocent on her part. The longer you allow this to go on the more you may be putting your marriage in jeopardy. Why does your husband have to talk to her so much texting and phone calls and emails, that is just too much contact, I could understand every once in a while but not as much as that. Not saying that your husband is going to have an affair but this is how alot of affairs start they start out innocent and then end up hurting alot of people because one or both parties decided to be selfish and thinking of their needs instead of their families needs. Just tell him it's not that I don't trust you but you don't know this woman and you don't know what she is saying to him to keep him wanting to talk to her. If you have anything to add click reply and let me know.