You have nothing to worry about.
Hi little one. As I don't know either yours nor your masters background, it's a little difficult to advise but I'll try anyhow. Were there any communications with this "old" pet, after you found out that you wanted to be together? If not you haven't got anything to worry about, as it's clear then that the other pet is history and it's all about you now. Therefore it's best if you try to act like nothing as he migh be very hurt, disapointed or even angry over the fact that you looked through his old mails. You know Curiosity Excited the Kat. So up with the spirits...