Here's the deal, man. You may think there is nothing but innocent flirtation going on. And, yeah, maybe you get to read some of the letters and stuff. But I guarantee there is more going on than you're willing to acknowledge. If your wife was really as happy with you as you seem to believe she would not feel the need to find approval with some other guy. There is obviously something missing in your relationship and I would seriously consider some form of relationship counseling. If you don't this thing WILL blow up in your face. I don't want this to sound like an ownership thing but she is YOUR wife. Take control and do something before it all crashes down on you.
Wake Up,
Justin