Husband and his female friend update

I really appreciate the advice and encouragement from everyone. I again talked to him about it & how I do not like him talking to her. It's like he's completely ignorant. I really think the female friend is up to no good. Yet he still won't stop responding to her. It's starting to cause an issue, because it bothers me so much. He keeps saying I'm being too jealous. I've never had a problem being jealous before, but this girl in particular doesn't sit right with me. If he didn't respond to her anymore I would be fine, but my husband is a people pleaser to a fault. He says that if I fully trust him then I wouldn't be so worried. It's not that I don't trust him, I just don't trust her. I also am puzzled about how she messages him every few months kinda thing. A while ago she said in a message to him that she would just delete his number & stop messaging him. So I thought that would be the end of it, but she's obviously still messaging him. I'm not sure if it's for attention or what. He keeps acting like it is no big deal & I'm just being jealous for no reason, he's being ignorant about it. I still have a strong feeling that she's up to something, whether she wants attention from someone that's not her husband or what I'm not sure, it's really not my business. However, I also saw some of their old messages from before I had my daughter & I got even more uneasy with their relationship. I think what bothers me the most is that he is totally ignorant about it.