Like an oak... never faulter or crack, strong as ever. Pffft... people are people and our curiosity will prevail.
First I would clear the air with my wife... I'd let her see the email (openess can be painful, but it does keep the lines of trust strong). Then I would probably stay in contact with her for a little while and let it fade away on its own (stay uncommitted and emotionless and the inevitable fade-away will happen).
This had actually happened to me. I received a letter, of all things, from an acquaintance that I had ceased a relationship with a few years before I had married. In the letter she wrote how much she had thought about our past relationship and that her current husband was a jerk. It was pretty freaky actually. She had put her phone number in the letter and a picture of her and her son. My wife was like, "WTF - who's this bitch?" Normal reaction type of thing... LOL
I had never told my wife about her. Didn't figure it important since I had quit seeing her a couple years before I started dating my wife. Well, my wife was pretty adamant about wanting to cut her tits off, shooting her twat, poking her eyes out, etc... so I did the most logical and safe thing (for me) at that time - I suggested that she write her a letter letting her know that I was already taken and I left it at that. Well, the next day, when I came home from work, I asked my wife if she had taken care of the problem and she told me she wrote her a letter. That was it... never heard from her again.
I had another past acquaintance that had called me at home once. She tracked me down when we were living in Florida. But, that's another story. It had a much better ending than the first.
Gotta love wives and how they handle the competition.