Very often you may find that you want to prohibit certain users from receiving mail from the internet. In the past, the solution would have been to give the specific users that are prohibited from receiving internet mail with non-resolvable SMTP domain addresses. Exchange 2003 now provides you with a feature that will NDR mail originating from the internet to users or distribution groups if they  mail was submitted anonymously. Anonymous authentication / mail will be the typical submission for mail originating from the internet.

To set the feature to require authentication to send to a distribution group, follow these steps:
 
1. Click "Start", point to "Programs", point to "Administrative Tools", and then click "Active Directory Users and Computers".

2. Right-click the distribution group, and then click "Properties".

3. Click the "Exchange General" tab.

4. Under "Message restrictions", click to select the "From authenticated users only" check box.

To set the feature to require authentication to send to a specific user, follow these steps:

1. Click "Start", point to "Programs", point to "Administrative Tools", and then click "Active Directory Users and Computers".

2. Right-click the user account, and then click "Properties".

3. Click the "Exchange General" tab.

4. Click "Delivery Restrictions".

5. Under "Message restrictions", click to select the "From authenticated users only" check box.

After this change, mail from the internet should effectively be rejected to the configured users as long as there was no authentication

- Ade Famoti