How to Block Users from Sending an Email Outside the Organization in Exchange Online

You can restrict users from sending emails outside your organization using mail flow rules in Exchange Online Office 365. Blocking outbound emails by creating a mail flow rule is the subject of this article so let’s get started.

Step-By-Step

5. In Exchange admin center dashboard, click rules under mail flow category.

6. Click plus sign and then click ‘create a new rule’.

7. Type the name of a new rule.
8. Choose ‘The recipient is located’ from ‘Apply this rule if’ drop down.
9. Choose ‘Outside the organization’ when a new window opens and then click OK.
10. Click ‘More Options’ to change other options.
11. Choose ‘Reject the message with the explanation’ from ‘Do the following’ drop down.
12. Type a message in the new window and click OK.
13. Click Save.

Now, the rule is in place and any Office 365 user won’t send an email outside the organization. When they try to send, they will get an email similar to following.