I would agree with mandarin mostly, with a tiny bit of match.com thrown in. I think jealousy is unattractive and will not endear you to your fiance, but maybe you don't really care about that. snooping is bad news cause it gives you information you can't use constructively - you will give your fiance the excuse to fixate on the snooping ("you don't trust me!") and as though it's somehow exculpatory for her inappropriate behavior. I think what your fiance did was not necessarily evidence that she's like the devil incarnate, or even a cheater, but you two are clearly having issues right now.
I think you have to ask yourself if you love her and still want to marry her, and for that matter, if you trust her. You know her better than any of us do. If you can answer those questions honestly, go with your instincts. I'm a woman, and I personally think that if she wasn't cheating, she was being manipulative, inflammatory and passive aggressive. Wedding planning can be irritating and stressful, and you guys might have been fighting over inane trivial bull-sh*t a lot lately, so this was her charming way of getting under your skin. She was flattered by the flirtation and attention of another dude and wanted to piss you off. She knew it would annoy you, just as she would have gone postal if you had gone out for 5 hours with another chick.
You should have waited til she came home and talked to her about her behavior. You should not have called the dude. I'm tempted to say you shouldn't have snooped, but it wasn't really snooping because she left the email in her inbox and her account logged in on a shared computer, so she knew at any given time you could see it. In fact, I'd argue in some subconscious way she left it to say "neener neener look here's a guy who pays attention to me." Whatever his marital problems, it has nothing to do (fundamentally) with you and your fiance. He's just one of many guys who might try to flirt and/or sleep with her, just as there will be many women who will flirt and/or try to sleep with you.
I think one possibility is that your fiance for whatever reason feels unappreciated or abandoned by you lately, or just pissy about your behavior, and wanted to be super immature about getting back at you. Call her out on it, but first admit you were wrong in calling the guy. Ask her WTF and if she really wants this wedding to happen. Say that in the future if she has issues with you or your relationship, act like a grown up, use her words, and have a conversation with you about it and resort to throwing tantrums.