How can I allow the administrator to see inside all users mailboxes?

From the SBS2000 Upgrade CD

To grant your administrative account access through Exchange System Manager to all mailboxes in a single database, regardless of inherited explicit denials, complete the following steps:
A. In the Small Business Server Administrator Console, expand Your Server (Exchange) to start Exchange System Manager. Expand Your Server, expand First Storage Group, right-click Mailbox Store (Your Server), and then click Properties.
B. Click the Security tab. (If the Security tab is not present, refer to Knowledge Base article Q259221 “Security Tab Not Available on All Objects in System Manager.”)
C. Ensure that your account has Full Control permissions.
D. Additionally, scroll to the bottom of the permissions list and grant your account Receive As and Send As permissions.