I am a long-time lurker of your thread. You fought long and hard for your M. Perhaps it might be a good time to establish boundaries around your M. Tell your H how you feel about the email contact using "I" statements.
"H, I would like to discuss the monster issue. It is too important to ignore this as it is making me feel hurtful and I want the pain to stop. I would like for us to get to the root of the problem which is the contact that is still going on between you two. For this M to heal and survive, all contact with the monster must cease immediately and onward. How about we write up a draft letter for you to send to the monster? I want us to work together as a team."
Here is a form letter that I've copied from another website:
No Contact Letter to Former Affair Parter -
<name>,
The relationship I had with you was thoughtless and cruel. It hurt many people, particularly my spouse who did not in any way deserve to be treated in that manner. I have recommitted to my marriage and am determined to make amends for the hurt I've caused and to be the partner my husband/wife deserves.
I am terribly sorry for all the pain I've caused, including to you and those around you. As I said, I was thoughtless and cruel. I hope that you will find healing and peace.
Because of the terrible offense to my husband/wife and the damage I have done to our marriage I am permanently ending all contact with you. I would ask that you respect my wish to regain my ethics and integrity and to heal my family and that you do not attempt to contact me in any way at any time.
My spouse has been told all the details of our relationship and s/he will also be told of any attempts at contact.
Once again, I am sorry for the pain I've caused to my spouse, my family, and to everyone else who has been affected by our actions. In order to protect my marriage and my wife/husband I must end all contact with you. Please respect my wish for no contact whatsoever in any form.
Sincerely (not love!!)
<name>
Ellie might be of assistance as her H wrote a similar no contact letter to the ex-OW. You can re-work this letter to suit your preferences. There is no need to issue an ultimatium, but to state very clearly and in no uncertain terms what your expectations are now that H is back home.
