ok, everyone says you shouldn't. some people say that if you do, then you deserve whatever it is that you discover. you know... the "serves you right" mentality.
Ok, but in a perfect world there would not really be anything there to discover would there? Also, if there was something to discover that they were doing and shouldn't be, wouldn't you at least want to know it? Or would you rather by another statistic of the old saying "the spouse is always the last to know..."?
Have you ever read their email? Did you regret it? Or did it end up being a big favor you did to yourself and opened your eyes about things?