How to block someone from sending emails outside the org in Exchange Online? Microsoft 365 tutorial 

To block someone from sending emails outside the organization in Exchange Online, you can use the following steps:
Log in to the Exchange admin center.
Go to Mail flow - Rules.
Click Add a rule and select Create a new rule from the drop-down list.
In the New rule creation wizard, under the Apply this rule if section, select The recipient - is external/internal.
In the pane that opens, choose Outside the organization from the drop-down list and confirm by clicking Save.
Under the Do the following section, select Reject the message with an explanation from the drop-down list.
In the Explanation box, type a message that will be sent to the sender when their email is blocked.
Click Next to review your rule settings.
Click Finish to create the rule.
Once you have created this rule, the user will not be able to send emails outside of the organization. If they try to send an email to an external recipient, the email will be rejected and they will receive the explanation message that you specified.
Additionally, you can also use the following steps to block a specific user from sending emails outside of the organization:
Log in to the Exchange admin center.
Go to Recipients -Mailboxes.
Select the mailbox of the user that you want to block.
Click Edit.
Under the Mail flow settings section, click Restrict user from sending mail and select Yes.
Click Save changes.
Once you have completed these steps, the user will no longer be able to send emails at all, including to internal recipients.
Please note that blocking someone from sending emails outside of the organization can have unintended consequences. For example, it may prevent them from being able to collaborate with external clients or partners. Before blocking someone, you should carefully consider the potential impact and make sure that it is the best solution for your organization.
by
Jojo Paul