Step 5 : Adding Incoming Domain Restriction

Click on the Edit tab, Create New Restriction, and Name your restriction  , Click on Edit button again and Now under Incoming Domain Restriction, Select Organization Domain only or Allowed Option to allow the incoming email from specific domains only , enter the domain name for example xyz.com and then click on + icon to add it

Step 6: Applying email policies to email account
Now to make it work you have to apply these email policies to an email account, for this go to the users tab, select the respective users, Click on mailbox settings , Email Policy and select the desired policy you want to apply

Step 7: Save Settings
Once you have added all the domains you want to allow emails from, click on the "Save" button to apply the changes.

That's it! You have successfully set up the "Approved Domains Only" feature in Zoho Mail. From now on, only emails from the approved domains will be delivered to your inbox, and all other emails will be blocked.

Same way you can add the domains to Blocked list also

Conclusion
In conclusion, Zoho Mail's "Approved Domains Only" feature is an excellent way to protect your organization from unwanted emails. By following the steps outlined above, you can easily set up this feature and ensure that your inbox is free from spam, phishing, and other malicious emails. Additionally, make sure to keep the list of approved domains updated regularly to ensure that you don't miss any important emails from new domains.