Technically you can’t. If someone is an Exchange Admin they can grant themselves rights to view any mailbox via Outlook. It is your admins professionalism which keeps them from doing this. If however you have reason to think that someone is looking at your email, bring this up with your management.
MrDenny is right. Exchange Admins require “god rights” on all mailboxes in order to troubleshoot issues in the event a user is unavailable or is having a problem getting into their mail. I cannot tell you how many times being able to view and work with a mailbox before calling a customer has saved an incredible amount of time. The end user is often away when I get the trouble ticket and that way I do not have to wait on them to be able to start figuring out the problem. But as mentioned this of course requires the admins are being ethical. Be aware that all your email belongs to your company, not you. Admins can be required by management or HR to view your mail without your permission or your knowledge.