Unfortunately they're not abusing their position--they're exercising the
abilities their position entails.
Speaking as an email administrator that has done this at the behest of
corporate management, there are often very compelling reasons for doing
this, and--as Gordon said--your email is not *your* email, it is the
*company's* email. It is their money that was invested into the servers,
the computers, the internet connection, and the software/licensing, so they
have every right to protect that investment to ensure it is being used in a
manner that is in line with the company's well-being.
If however you feel this is done specifically to you for some
non-professional reasons, then the only thing I can suggest is to go through
the proper channels to inform management of your concerns.
Joe