Office 365 Advanced Threat Protection, Anti-Phishing Policy : SafeTitan Help Center

Logon to your Office 365 Security Admin Center portal.
From the Microsoft Defender page, click on Policies & rules, then Threat policies, and finally click Anti-Phishing.
Click on the Office 365 AntiPhish Default (Default) Rule.
On the following screen, click on Edit Protection Settings.
Click on Manage 0 trusted sender(s) and domain(s)

Note: There may be previously added senders or domains for your organisation, which would mean that the above link would not show a zero (0) value.

Here you can add trusted / known email addresses and domains.
This will allow you to override the anti-phishing policy for added domains only.
On the screen that follows the Sender tab will initially be highlighted. Ignore that and click on the Domain tab.
Click the plus icon and then add each of the following domains listed below:

Click the plus icon, and enter the name of each domain one by one. As you enter the domains, they should appear as a clickable item once they’re valid.

Do not select the Add Domains button until all the domains are added.
When all the domains have been added, click the Add domains button.
You will see a list of domains. Please ensure that you've added all the domains correctly.
Click the Done button.
Finally, click Save.

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