How to Restrict Users from Receiving an Email Outside the Organization in Exchange Online Office 365

Step-By-Step
 

1. Open office.portal.com.
2. Log in with the global administrator account.
3. Go to admin center.Expand Admin centers and then click Exchange.
4. In Exchange admin center dashboard, click rules under mail flow category.
5. Click plus sign and then click ‘create a new rule’.
6. Type the name of rule
7. Choose ‘The sender is located’ from ‘Apply this rule’ drop down. When a new window opens, choose ‘Outside the organization’ and click OK. 
8. Choose ‘Delete the message without notifying anyone’ from ‘Do the following’ drop down.
9. Click Save to save the new rule.