> As of February 28, 2021, the specific "Outlook.com Premium" and "Ad-Free Outlook.com" subscriptions no longer include the ability to use a custom domain with Outlook.com.
> To continue using a personalized email address and custom domains with Outlook.com, you will need to subscribe to Microsoft 365. Learn more about Premium features in Outlook.com for Microsoft 365 subscribers.
https://support.microsoft.com/en-us/office/get-a-personalize...