Emailing list

my husband and me are around 30 and have been married for 6 years. our relationship has been rocky but the feelings are still quite strong and so we have held it together. We both work for the same place but in different departments and different shifts. A female coworker of my husbands who is a bit younger and thinner(thats my insecurity) has recently started emailing my husband and becoming friendly with him. She seems like a nice girl and I want to believe that this is all just being nice- but It still bothers me that this is happening. It started with them making each other CD’s because my husband listens to his ipod at work and i guess they started talking about the music that they like. Then she emailed him to ask him what he thought of it and if he had a favorite song on it. I didnt know until I found the CD with a nice typed out list of all of the songs. Nothing too romantic but it freaked me out. Then I stumbled upon an email when he didnt know- It was her asking him to go play paintball with her-and the invite was to noone else just him. He hadnt even opened her emails but Im sure they talk at work. She also invited him to her birthday party- she also has a boyfriend-but I know this doesnt always matter. The thing that bothered me is he didnt tell me-once again I just found the invite and he said he might go when I asked. I dont want to be a jealous neurotic wife but this keeps tugging at me. I think my husband is great and honest and they may be friends as coworkers but I think anything can happen if the conditions are layed out, especcially if they get along well and are hanging out and discussing personal things over private email like likes and dislikes. Is it flirting? is it inappropriate? I let him know I was uncomfortable with it and he says I have nothing to worry about.