Here’s a step-by-step procedure on how to whitelist a domain name using Office 365 Security and Compliance.
First, login using your Office 365 administrator.
Go to Office 365 Security and Compliance dashboard or using this link (Anti-spam policies – Security & Compliance (office.com))
Click on Anti-inbound Policy to whitelist incoming traffic.
Scroll down till you see “Allowed Domains”.
Click on Edit to add domain.
Click on Allow domains.
Add the domain name and save.
Wait for the servers to synchronize and you’re set.