The very first thing I would consider was that there was some malicious person trying to bring trouble to my marriage by telling me lies about my spouse. I would be worried about it though, because even if it were a lie, it would indicate that there was someone who really wanted to make trouble in the marriage. I hope you are not asking this question because of something that has happened in your own marriage.
If you are suspicious of your husband show him the letter and observe his reactions. After ten years together you ought to be able to know if he is lying.