An organization has just signed up for Microsoft 365 E3 with the initial domain 'contoso.onmicrosoft.com'. They need to create the first user accounts. What will be the default email address format for these new users if no custom domain is added yet?
The initial domain created when a Microsoft 365 tenant is provisioned always uses the <tenantname>.onmicrosoft.com namespace, and contoso.onmicrosoft.com is the default UPN and email domain for all new users. This domain is automatically reserved for your tenant and cannot be used by any other tenant, so assigning user@contoso.onmicrosoft.com is the only valid option listed without additional configuration.
Why this answer
When a new Microsoft 365 tenant is created with the initial domain 'contoso.onmicrosoft.com' and no custom domain has been added, the default email address format for new users is user@contoso.onmicrosoft.com. This is because the onmicrosoft.com domain is the default tenant domain provisioned by Azure AD, and all user principal names (UPNs) and email addresses are automatically assigned this suffix until a custom domain is verified and set as the primary domain.
Exam trap
The trap here is that candidates assume the email address will automatically match the organization's public domain name (e.g., contoso.com) without realizing that a custom domain must be explicitly added and verified in the Microsoft 365 admin center before it can be used for user email addresses.
How to eliminate wrong answers
Option B is wrong because 'contoso.com' is a custom domain that must be purchased and verified via DNS TXT records before it can be used for email addresses; it is not automatically available. Option C is wrong because 'microsoft.com' is Microsoft's own corporate domain and cannot be used by any tenant. Option D is wrong because 'contoso.microsoft.com' is not a valid domain format for any Microsoft 365 tenant; the default tenant domain always uses the pattern <tenantname>.onmicrosoft.com.