God, this is really dangerous. Is your ex angry with you and trying to get you into trouble? People have been sacked for sending sexually explicit emails, I'm afraid. So you need to put it on record somehow that these replies weren't from you. As you probably know, deleting the emails is no good as they can be retrieved even after the user has deleted them. Can you prove that at the time these replies were sent, you were not at your computer - were they sent at times when you were out of the building, or in meetings?
But putting it on record is itself dangerous as it draws the authorities' attention to what has been going on... and if your dh is sacked, obviously that also affects you.
Do you have a union you can speak to in confidence, who can advise you on protecting your position?
You need specialist advice from someone who understands all the implications of employment law. I hope there is such a person on MN. If not, and if you don't have a union, you may need to see an employment lawyer - one who has a background in working for employees, not employers, presumably. Do you have legal insurance via your house insurance? Check out if so and if it would cover advice about employment. If you are worried about paying for a lawyer, may be worth talking to your local citizen's advice bureaux, don't know if their lawyers could advise you themselves but they may be able to suggest firms of lawyers that do pro bono work?