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 connection filter tab.
Step 7: Double-click on the default policy or click on the edit icon.
Step 8: Switch to connection filtering.
Step 9. Click on the Add icon respective to the IP Block list section.
Step 10: Enter the IP address and click OK.
Step 11: Click Save.