How to mark empty emails as spam in Office 365?

Step 1: Sign into your Office admin account via https://portal.office.com

Step 2: Navigate to Exchange admin center.

Step 3: On the left pane, scroll down and click More features.

Step 4: Click open under Permissions.

Step 5: Click Protection on the left pane.

Step 6: Switch to the spam filter tab.

Step 7: Double-click on the default policy.

Step 8: Click on advanced options.

Step 9: Under the Mark as Spam area, in the Empty messages section, select On and click Save.