In order for anyone to access mail in your mailbox on an Exchange server, they would essentially have to be logged on as you or with an account that has the same permissions to your mail as you do. But someone other than you could grant the necessary permissions. For Exchange 2000 and 2003, an Exchange administrator could create an administrative account that has permissions to all mailboxes on an Exchange server (see KB268754 and KB262054 at http://support.microsoft.com). Once logged on as that account, they could access all mailboxes in the organization from their own machine. If your mailbox resides on an Exchange 5.5 server, then there is an account by default that has rights to every mailbox and that account is called the Exchange Service Account.
This was first published in September 2004