DS is not open to R - she will not cut off contact with OM which is a deal breaker for me. At this point I see no choice but to "move on".
Ok. In this case, if you want to email him you can say something like this:
"OM's Name,
My name is DSSM. I am X's husband. I am aware you have had sex and have having an ongoing affair with my wife. I know this because X openly admitted it to me.
I am writing to advise you to get tested for STDs. As you
know, X has not exactly been faithful and it would do you a world of good to get the full range of tests done and then go back to re-test in six months. I am telling you this as a caution to your health.
I am sorry we had to learn about eachother under these circumstances.
If you have any quetsions, you can contact me.
DSSM"
If he responds, let a few days go by without writing back.
This does a couple different things:
1. It lets him know you are fully aware he helped betray your marriage & the affair ain't a secret anymore! Exposure is scary!
2. It puts your wife's and his relationship into question. Notice, you didn't lie in the letter above, you simply stated "Wife hasn't exactly been faithful" so he is wondering if she has fcked around on him too. The more vague you are, the better
3. He is now freaking out because you just told him to get tested for STDs without really saying why ... he is now wondering if you have something, which means she might have something, which means he might have something. And, to add more gravity, you seal it with a kiss when you say "get re-tested in 6 months."
4. Your approach was in a calm/caring manner, not accusatory or psycho. THIS
WILL MESS WITH HIS HEAD. Trust me. He'll think, Wow, that guy could bit my a$$ yet he was nice and genuinely concerned which means, holy sh*t... she is prob cheating on me too with someone else AND now I may have a gross STD. Your sounding polite and genuinely concerned is going to throw him for a loop. Big. Time.
Win-win, all around