Need advice

I'm in a weird situation that has my anxiety up. My SO put his Apple cloud email on my phone because I still have an iPhone and he switched to android. He has openly said he doesn't care if I look through it or his phone because he has nothing to hide and I don't go through it because I trust him. I was on my phone and dropped it and when I picked it up the email app was open. Both our emails are on the same app, your just switch inboxes. I looked at it briefly thinking it was mine and saw strange messages sexual ones from Craigslist. It freaked me out for a second but then I saw the dates and details. They were in his sent inbox and and all from before he and I were together and he was single. So they are not cheating messages. The parts I saw are freaky stuff and I won't judge on that but now I'm curious if he did these things or was just internet trolling. It's nothing that would cross a line from what I saw but now I'm wondering did he meet up with these people and do these things. We were friends before so I know he used to be a player. He has been with many women in the past and used protection and gets tested regularly. He is super attractive and charismatic and he was young and single so if he did meet up with them, it's not like he did anything wrong. I wish I had never saw the messages because the urge to actually go through them and read them is eating at me. I glanced at the inbox but once I realized they were old and his I closed it. I am going to tell him what I saw. Do I look at them or not though? Curiosity is killing me but ignorance is also bliss. We have a great relationship. Would looking at his past change how I see him?