greetings to you... forgive me for getting straight to the point but yes, i think you've made a mistake by checking out his mail without permission first. although you do have the right.
just imagine if he did the same thing to you... you might also be disappointed because he was trespassing. so please keep in mind that we must respect other people's privacy, even when we're married.
if you do want to know about his past, just ask him politely, with the intention to just get to know him better. he may not tell you right away because trust also needs time to develop between the 2 of you.
i don't think it's a fatal mistake. just tell him you're sorry, and i'm sure he will appreciate your apologies. don't force him to talk about it right away, though. give it some time before you ask him the same question. have faith in him and he will do the same with you. men need our trust. that's what i learned myself.
hope that will help you out a little bit. have a nice day... :)