Not knowing if you have accessed her email account improperly, I would think this is not a crime? It may be though. Regardless, the threat to prosecute you criminally in a civil action is itself actionable and may come back to bite her. I'd tell your attorney and s/he may be able to use this threat to help you substantially. If you do not have one, you should...
The threats she mace could even help you in the child custody case, depending on the judge.
The answers on this discussion board are general in nature and NOT intended as legal advice. Responding to questions does not constitute an attorney-client relationship. Always see a lawyer about your individual situation.
Sign up to receive a 3-part series of useful information and advice about child custody law.