Is it OK to snoop on your partner's emails ?

Nope. Most people keep their email on their cell phones, so that level of snooping involves picking up their phone and rifling through it.

I used to share a computer with the 3rd ex, and got quite a surprise when I opened it up and found all his teenage porn girls opening their legs at me. I mentioned it to him, and suddenly found out that he wanted to be the ADMIN on our shared computer, and I was locked out of anything he was viewing.

But he had no such qualms, and used to rifle through my bag, and searched my car and removed my duplicate house keys.