Were it me, if I bothered to read the email at all, I'd just shake my head at the sour grapes the ex is sending and perhaps chuckle at how transparent he is.
As you already know about your husband's past cheating and that he's currently seeing the gf, then there's nothing of note in the ex's message. He's acting out against people he sees as having hurt him and nothing more.
The thing is, he can only hurt you in that fashion if you allow him to do so. If you've chosen to move forward after working things out with your husband, then move forward and leave the past behind.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.