Hi Optimist,
I agree with KAW that you should try to hold on and see if he offers an explanation first. Maybe leave the mail out so that he knows you've seen it, but don't be obvious that you're waiting for some kind of response. Or could you not be home when he comes home? Opt, this will be a challenging one. I know your emotions are going crazy. Things have been going so well for you, remember "will it bring him closer or push him farther away?"
Also, I get the impression that OW really depends on your H to help her with a lot of life stuff that she doesn't know how to do. Also, it seems like he's starting to feel resentful towards her about it. Maybe this is something he was helping her with before and since has removed himself, hence the rebate? Hang in there!
PS Can you stop by mine again. I feel like I screwed up.