Here's a question that got asked by a reader today. They were wondering how to configure their Exchange server so that all inbound emails sent to invalid addresses for their domain got redirected to the administrator.
I don't think that there's a mechanism to give them exactly what they want, however, Exchange (both 5.5 and 2000/2003) allow for Non-Delivery Reports (NDRs) to be copied to an administrative mailbox. To me, this accomplishes much the same thing, unless you don't want NDRs ever to be sent out by your server.
You can find a relevant Microsoft KB article here: http://support.microsoft.com/kb/294757/
I test turning off sending of NDRs to the internet and the admin account still continues to receive NDRs even if they aren't sent to the original sender.
Anyhow, to set this up in 2003/2000, go to each Default SMTP Server for each server in your site. Go to Messages tab in Properties and fill in the proper email address in the text box for "Send copy of all Non-Delivery Reports To".
If you are using Exchange 5.5, the same functionality is built into the Internet Message Service. Go to the IMS properties for your site and select the Internet Mail tab. Set up the Administrator's Mailbox and then click Notifications to set up which NDRs you want the mailbox to receive (or all if you wish).