Block any message going out to a specific set of email addresses.

Create a recipient group

• Navigate to Services > Platform > User Groups > Create New Group 
• Enter a name for the group (Example: Block all Outbound emails to specific addresses)
• Enter external recipient email addresses in the New user's box and click Add
• Save and exit

Create the block policy 

• Select Services > Data Protection > New Policy 
• Enter a name for the new policy (Example: Block all Outbound emails to specific addresses)
• Select Apply To: Outbound email only
• Execute if: All rules are met
• Change Action to "Block and Delete" 
• Beside Action, check the box for "Stop evaluation of lower priority policies". Checking this box ensures that if an email is blocked due to this policy, no other policies with lower priority are processed.
• Administrator email: Use the default administrator address or select Use custom 
• Decide on your use of notifications for the policy. By default only the admin gets notified when the policy is triggered.

Add rules to the policy

• Click Add Rule 
• Execute if: Any conditions are met 
• Add a condition > Recipient group > Browse for a group > Select the recipient group created earlier (Block all Outbound emails to specific addresses) > Add
• Select "is in any of the selected groups"
• Click Save to save your changes