Sometimes an organization security policies will demand to prevent email users from sending messages to certain external domain names. This can be achieved with Exchange Server 2010 using a Transport Rule.
Procedure:
- Open the Exchange Management Console and navigate to Organization Config/Hub Transport.
- Start a New Transport Rule. Give an appropriate name and description to the rule.
- Choose conditions of "From users that are inside the organization" and "When a recipients address contains specific words". Click on "specific words" and add the domain name you want to block, for example "@fabrikam" (without quotes). You can enter several domain names in this list.

- Click Next and choose actions of "Send rejection message to sender…."

- Enter a rejection message and an enhanced status code so that the sender or the IT admins can easily tell why the email was rejected.
- Complete the Transport Rule wizard and test the new rule by trying to send an email to that domain name.

- You should now receive a bounce message from the Exchange server with the text that you configured.



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