Slander is using disparaging words SPOKEN about another.  Libel is using disparaging words that are WRITTEN about another.

So, this may be Libel, because it was written.  Truth is a TOTAL defense, so if what was written can be proven to be true, your ex-wife cannot sue and recober.

If your divorce is proceeding now, your wife MUST bring this claim in the divorce action - she cannot bring a separate action if your divorce is now pending.

If your words were NOT TRUE, you may still be ok if your words wre only your "opinion". If you made a statement of opinion as opposed to fact, youre statement may not support a claim for defamation.  Another defense exists if you merely transmitted a message without knowing its content; that's called "innocent dissemination".

All in all, you should see an attorney to discuss the facts of this case.