Why are you going through her e-mails?
Adults don’t usually go through each others’ e-mails. They’re private. Would you open her snail mail? (I hope the answer is no.) Do you go through her texts regularly? How about her landline phone bill to look for suspicious phone numbers? This isn’t looking like a happy engagement destined for a solid and loving marriage.
If you’ve created a climate in the relationship that is controlling, and she feels that the only way to have friends is in secret (from you), then you’re headed for a disaster in your relationship. Back off and let her have her privacy, if that’s the case. If she has a male friend you’re threatened by, explain your feelings to her. Get them out on the table now — before you’re married. If she denies having the friend, and you have proof that he exists, your fiance is lying to you! About another man. Time to exit.
On the other hand, if you’re going through her e-mails because you already suspect her of cheating on you, and that’s the sole purpose for the detective search, then, you shouldn’t be engaged to her. This is one of the most intimate times in a relationship, when two people have agreed to spend the rest of their lives together. If you’re suspicious of a third person in the relationship (or more than a third person), then back out of the engagement, and move on to find a woman who you can trust and respect as well as love.
Regardless, the secrecy in your relationship is a barrier to intimacy, so until you get the issues out on the table and confront them as a couple, and work towards solutions together, you’re always going to feel like you’re working against her, and she’s working against you.