Please help!!!

My wife just gave me an old cell phone to use. I opened up the email app on the phone not realizing it was her account. When I went to the sent mail there was a response to a Craigslist personal for a *full body message". When I clicked the origin post it took me a page from the casual encounters category. Which upon further investigation is clearly an online hook up. Nothing more than casual sex. I showed my wife what I had seen and her answer to me was simply..... I don't remember sending that. She claims that she has no recollection of the email and that if it was real didn't I think there would be a full conversation rather than just one sent email. By the way the email was titled face shots and the message was "I saw your add. Have any face shots". That's it. Am I crazy for wanting to believe it was as simple as she really didn't send that email or is there for real cause for concern and major questions. Please help me if there is any chance she could not have sent that message.