To allow a complete domain or specific sender, we need to modify the inbound spam policy.

From Microsoft Security Center: https://security.microsoft.com/

1. Click on Policies & rules
2. Select Threat Policies
3. Open Anti-Spam (it can take a couple of seconds to load the policies)
4. Click on the Anti-spam inbound policy (Default)

5. Scroll all the way down in the fly-out and click on Edit allowed and blocked senders and domains

6. Click on Allow domains
7. Add the domains that you want to whitelist
8. Click Done and Save