Exchange 2016 restrict users from sending email to external recipients

1. How to restrict users from sending emails outside of organization Microsoft Exchange?
2. How to restrict users to send and receive external messages in Exchange Server 2016?
3. Can you restrict someone from sending you mail?

How to restrict users from sending emails outside of organization Microsoft Exchange?

Block emails sent outside your Office 365 organization

Sign 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.

How to restrict users to send and receive external messages in Exchange Server 2016?

Expand Recipient Configuration

Click on Mailbox. Double click on the mailbox that you want to restrict. Click on Mail flow settings tab. Click on Message Delivery Restriction and click on Properties.

Can you restrict someone from sending you mail?

At the time of mailing, you can: Request Restricted Delivery service at the time of mailing by asking the USPS clerk or by marking the mail "Restricted Delivery."