Block Mail to Recipients Outside of your Organization

We recently announced that we've started work on a  Kerio Server Migration to Office 365.  One of our clients gave us test data from a few departed/terminated users to test our code.  It's easy to test in our Exchange on-prem sandbox to ensure no "external" email gets sent to their users -- we unplug the Ethernet connection to the Router. It's a little more complicated in Office 365, but not all that difficult.  Here are the steps:

In the Exchange Admin Center, under Mail Flow, Rules, click the "+" sign to create a new rule.

• Name the rule.  We called it "Block Mail sent to External Email"
• Select the option from Apply this rule pulldown: "The Recipient is Located...."
• Select the option "Outside The Organization" from the subsequent pulldown that the recipient is located 
• Select "Reject the message with the explanation" from the pulldown "Do the Following..."
• Enter a message (optional):  We entered "The message was not sent. The Recipient is located outside the company."
• We chose to Enforce the rule, and finally
• Saved it

  Here is a screen shot:

So now let's say a user tries to send email outside your domain.  They will be informed that is an unsanctioned action with this message: