LOL

come on now man, why are you spying on the ex?? Unless you have her permission to view her email, I would suspect this to be invasion of privacy. I am pretty sure that if she found out you were reading her mail, she would be within her legal rights to sue you for invasion of privacy. So when you ask how wrong it is to read your ex girlfriend's email without her permission, I would say illegal (and kinda lame). Just don't let her find out =\

[EDIT]
I'm gonna give you AntiPoints for having the balls to ask if it is ok to read your ex-girlfriend's email
[/EDIT]