Restrict Office 365 users to internal email only

Question

As the title suggests, I have a group of users that I want to prevent from sending external emails.

I have created an Office 365 group, and added the required users to it. As administrator, I am the owner of the group, and therefore must be a member.

The problem is that I want the administrator account to be able to send external emails!

I could make one of the users the group owner, but then as owner they could add/remove users, which is obviously a security issue.

What is the best way to get around the problems I have described?

Answer 1

Use a mail flow rule through the Office 365 administrator portal.

Steps taken verbatim from: https://www.itprotoday.com/identity-access-management/how-block-users-sending-email-outside-organization-exchange-online

1. Log on to your office.portal.com as the administrator
2. Go to admin center
3. Expand admin center and click Exchange
4. Click "Rules" under "Mail flow" category
5. Click "+" to create a new rule
6. Fill in the rule name
7. Choose "The recipient is located" from "Apply this rule if" drop down.
8. Choose "Outside the organization" when a new window opens and then click OK.
9. Click "More Options" to change other options.
10. Choose "Reject the message with the explanation" from "Do the following" drop down.
11. Type a message in the new window and click OK.
12. Click Save.