How to whitelist a domain in Gmail?
1.Log into your Gmail account.
2. Click on the gear icon at the top right corner.
3. Click on the “See all settings” selection.
4. Go to the “Filters and Blocked Addresses” tab, then Click on “Create a new filter”.
5. Now, go to the ‘From’ box and type in the domain or email address you’d like to whitelist, then click ‘create filter’ at the bottom.
6. Check the box “Never send it to Spam” followed by the “Create filter” button. This will ensure emails from that domain never go to spam.
You may also want to check “Always mark it as important”.
8. On the same page, categorize the domain as primary to make sure it lands in the primary tab. Otherwise, it can go to promotions or social tabs and you may not read it.
9. Finally, you’ll see the confirmation pop-up “Your filter was created.” You’ll also be able to see the filter as shown below.
Et voila! — Now Gmail will never send emails from your most important senders to the Spam folder. All you have to do is whitelist those domains.
https://gatlabs.com/blogpost/how-and-why-to-whitelist-a-domain-in-gmail/