How can I restrict a user from sending to a specific external email address? I am running exchange server 2003 SP1. I am not sure if this is possible.
Assuming you're trying to stop email from being sent to an external address, you can create a mail-enabled contact object in your Active Directory and assign the external email address to that contact object (optionally hiding the contact from Exchange address lists if you so choose). Once you've created the mail-enabled contact object, access its properties and specify the mailboxes of the senders who are not permitted to send email to that address on the "Exchange General" tab under "Message restrictions" with the "From everyone except" radio button selected.
If you're trying to stop messages from being sent by a sender to an internal address, just add the unwanted sender under "Message restrictions" on the object to which that internal email address is already assigned in the Active Directory.
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If the destination email address is one of your Exchange addresses you could simply use Delivery Restrictions option for the account (in ADUC, view the properties, Exchange General Tab, Delivery Restrictions)