If my W sends emails (or any other type of communication) to anybody inquiring about sexual relationships, I'd have a big problem, no matter what the recipients' gender might be.
Assuming you're supposed to be exclusive with one another, why not just leave him? Alternatively, you can give him a 2nd chance: 'If you send one more inquiry, I'm outta here.'
What's keeping you from ending your relationship with this guy.
When I started on SI, I thought my sitch was very rare. It turns out it's not so rare, but what's more important, our WS's infidelity counts a lot more than the ap.
This thread doesn't get a lot of activity, so you might consider raising this issue over on R or G. I think you'd get more responses there.