I have no issues with LT reading my texts or e-mails. Why? Because I have broken boundaries in the past. So I have a track record of being naughty. She is within her rights to be suspicious. It helps to keep me honest, and in MY mind, I deserve that. LT rarely ever checks up on me anymore though. But I haven't given her any reason to, in the last few years, either. So I've learned my lesson and have kept EVERYTHING above the board.
So, did your hubby have a track record of breaking the boundaries? If not, then WHY were you snooping? (not that it was "wrong" of you)
Also, Keep an open mind, and gently let him know that you know about it. You keeping silent about that, is almost as bad as him doing it.
Just my 2 cents.