Does anyone think its cheating talking to others via email and not telling you?

My husband and I have been married since 05 and over the years he has talked to ex's via email or myspace, and now some strange girl he claims he doesnt know in person (Im still not sure how he knows her), he doesnt talk or text them on the phone, but I think because he is keeping it from it is still a form of cheating. How hard is it to say that you are talking to a female friend, if she is just a friend?? But this has happened repeatedly, Im to my breaking point and done with it! Help!