Maybe if reading his emails to her upsets you, you shouldn't do it? It seems a little invasive to me anyways, though I can certainly see how you'd have some trust issues going on right now.
Do you know T at all personally, could you see you and he ever becoming friends? Many people find that helps because it allows them to see the other person not as competition but, perhaps in time, as a collaborator in making the same person happy. It also build trust, which cuts down on fear and jealousy.
If the NRE is too much for you right now, maybe this is another area where she could go at your pace by just not bringing him up all the dang time. I'm not advocating am emotional version of don't-ask-don't-tell, but I don't see that there's any need for her to be telling you whenever he pops into her head, I think that'd get old no matter what the situation, but especially considering that you're healing from being cheated on!
Do you think you may ever be interested in an outside relationship of your own? I'm not saying it'd be the best idea right now, by any means, but when everything is on a more even keel it might be worth exploring if it feels right.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.