Is it ok to open someone else’s mail?

Is it wrong to open someone else’s mail? I have always been taught by my mother that under no circumstances do you open someone else’s mail. It’s rude and not to mention illegal right? Well my friend told me the other day that she opens her other half’s mail. I couldn’t believe it when she told me. When I asked her if he opens her mail too she said no. He seems perfectly fine with it but to be honest he’s a very laid back person and he’s also quite naive too whereas she’s the complete opposite! They say opposites attract right?

What’s your opinion on this? Personally, I think even if my other half gave me his verbal permission to open his mail I wouldn’t as I’ve been brought up that it’s wrong.