Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello Jock,
Thank you for posting in our forum. Generally, we don’t recommend customer to use domain on 2 separate email services. In your situation, one way I think you may consider if Pilot Microsoft 365 from my custom domain this solution is suitable to you. Another way is you can continue to use custom domain on current email service, use Microsoft 365 fallback domain (contoso.onmicrosoft.com) on Microsoft 365 account.
Best Regards,
Etta