E-Mail Privacy
There is nothing in this country to protect the privacy of e-mail. I would imagine that the person who owns the e-mail account will be held accountable for any and all messages sent and recieved. By giving out the password to the account, you are stating that you agree with or know about all subject matter regarding the individual messages. Therefore, if you give someone access to you e-mail account and they send a death threat it will be considered a threat from the person who owns the account.
This is only my opinion, if I were a juror on a case involving something like this: I would be hard pressed to believe that anyone besides the owner of the account was guilty. After all, why would anyone "loan" their e-mail account to anyone in the first place? A person can get their own e-mail account for free on numerous web sites!