are you sure about your supervisor reading your personal emails?
IT may backup all communications, but it doesn't mean IT will turn over all your info to your supervisor everyday.
nobody does that unless you abuse the system. i don't think you do. Unless your supervisor wants to fire you, s/he won't read your personal communications. The reason to do so is to collect the evidence, in case you file a lawsuit for wrongly termination.
do you have performance issue? that might be the cause. i seriously doubt yahoo mail or gmail will cause any trouble.
