Step 1: Log in to your Admin Panel using https://portal.office.com.
Step 2: Select the Admin.
Step 3: In the lower left navigation, select Show all option.
Step 4: Under Admin Centers, click the Security.
Step 5: Click on Policies & Rules.
Step 6: Select the Threat Policies.
Step 7: Click on Anti-Spam Policies.
Step 8: Select the Anti-Spam Inbound Policy (Default).
Step 9: Scroll down and click on Edit Allowed and Blocked Senders and Domains.
Step 10: Click the Allowed Domains.
Step 11: Click the Add Domain.
Step 12: Enter the Domain Name.
Step 13: Click Add to allow the domain.
