Whitelist incoming domain

Login to the Google Workspace Admin Console and go to Apps -> Google Workspace -> Gmail -> Spam, Phishing and Malware. 

Click on the Configure button next to Spam.

Add a description (e.g. Allow domains).

Select Bypass spam filters for messages from senders or domains in selected lists.

Click on Create or edit list.

Make an address list with the domains you want to whitelist.

Go back to the spam window, click Use existing list and select the list you just made.

Click save.

References

https://support.google.com/a/answer/60752

https://support.google.com/a/answer/2368132