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Some customers want to block all mail sent from inside the company to a specific email address outside the company. To do this, create a contact in the Active Directory and give it the email address to which you want to block mail. Then, on the “Exchange General“ tab of the contact's properties*, you can configure who is allowed to send to the contact and who isn't. If your intent is to keep everyone from being able to send to this contact, set a message restriction that allows only a dummy user to send mail to the contact (this is much more efficient than creating a huge list of users and setting the contact to reject mail from users in that list).

* You may have to wait for the Recipient Update Service to process the contact before you can see it.

- Daniel Longley

2 Comments
Not applicable
More often I get requests to block mail to an entire domain. If it is a domain where we don't want people to go in general, I will add a zone to DNS for that domain and create an Mx record pointing to a non-routable IP. Guess one could also write an event sink to do this. :)
Not applicable
How about just blocking by IP Address? Is that possible?