I'm thinking of going through my bosses email.
Why would you want to go through your bosses email?
I am interested in computer security. I want to practice on my bosses email
It's disrespectful and an invasion of privacy to go through someone else's emails. You should do your best to not invade someone's privacy like that.
I need to know what's on his computer for my own safety.
That sounds like a good way to lose your job. I understand you want to learn through experience, but doing it illegally will betray your bosses trust. Ask them if you could borrow a fake email or someone who is willing to be the guinea pig.