Step 1: Log in to your Admin Panel.
Step 2: Select Admin.
Step 3: In the lower-left navigation, click Show all.
Step 4: Under Admin Centers, click Security.
Step 5: Click Policies & Rules.
Step 6: Select Threat Policies.
Step 7: Click Anti-Spam Policies.
Step 8: Scroll down and click Edit Allowed and Blocked Senders and Domains.
Step 9: Click Blocked Domains.
Step 10: Click Add Domain.
Step 11: Enter the Domain Name.
Step 12: Click Add to block the domain.
