Quickest way is to know some background information, then try a password reset and try to pass the security questions.
Otherwise easiest way is to send a spoofed email, with a attachment that is infected by a trojan/rat, and hope the guy opens it, if he does you get all of his passwords.
If that doesn't work, then send 5-10 emails to that email, from 5-10 different email account, and try to pass the security by answering the questions (that you have knowledge about the account)
There you have 3 different ways