"When you sign up for Microsoft 365, Microsoft provides an onmicrosoft.com domain—your fallback domain—in case you don’t own a domain or don’t want to connect it to Microsoft 365 (for example, tailspintoys.onmicrosoft.com). Your fallback domain serves as a default email routing address for your Microsoft 365 environment. Even if you use a custom domain (such as tailspintoys.com), if that custom domain is deleted from your Microsoft 365 environment, the fallback domain ensures that your users’ email is successfully routed.
Common reasons customers change their fallback domain include:
Not knowing the company name when they first signed up for Microsoft 365.
Wanting appropriate login account names for users.
Changing how SharePoint URLs look when creating new sites (SharePoint URLs are based on the fallback domain name).
Here’s how you can add and replace your onmicrosoft.com fallback domain in Microsoft 365:
Add a New onmicrosoft.com Domain:
In the Microsoft 365 admin center, go to Settings and select Domains.
Choose your existing onmicrosoft.com default domain.
On the domain properties page, select Add onmicrosoft domain.
Enter the name for your new onmicrosoft.com domain.
Verify the spelling and accuracy of the domain name.
Note: You can have up to five onmicrosoft.com domains, and once added, they cannot be removed1.
Make Your New onmicrosoft.com Domain the Fallback Domain:
In the Microsoft 365 admin center, go to Settings and select Domains.
Choose the new onmicrosoft.com domain you created.
On the domain’s property page, select Make fallback domain.
A message will confirm that your fallback domain has changed to the new domain1.
Remember that these changes affect the entire tenant, so ensure you have the necessary permissions as a Domain Name Administrator or Global Administrator. 🌐🔑"
also, didn't vcsy have a fallback server ip/patent?
