MS-102 Deploy and manage a Microsoft 365 tenant Practice Question
A company has just signed up for Microsoft 365 Business Standard without adding a custom domain. An administrator needs to create the first user accounts. What will be the default email address format for these new users?
⚠ Common exam trap
Candidates often confuse the default `onmicrosoft.com` domain with the generic `microsoftonline.com` domain used for Azure AD authentication, or assume a custom domain like `contoso.com` is automatically assigned, leading them to select A or D instead of recognizing the tenant-specific subdomain format.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
username@<tenantname>.onmicrosoft.com
When a Microsoft 365 tenant is created without adding a custom domain, the default domain is the `<tenantname>.onmicrosoft.com` domain. New user accounts are automatically assigned an email address in the format `username@<tenantname>.onmicrosoft.com`, as this is the initial domain provisioned for the tenant. Option C correctly reflects this default behavior.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
username@contoso.com
Why it's wrong here
The initial Microsoft 365 tenant has no access to contoso.com until you add and verify that domain as a custom domain in the admin center. At the moment of signup, the only domain configured for user email addresses is the system-provided tenant domain. Therefore, username@contoso.com cannot be used as the default SMTP address for newly created users. Verification of DNS records is required before contoso.com can become a valid email suffix.
- ✗
username@onmicrosoft.com
Why it's wrong here
The actual initial domain for a Microsoft 365 tenant is a concatenation of your tenant identifier and the onmicrosoft.com suffix, such as contoso.onmicrosoft.com. Username@onmicrosoft.com omits the tenant-specific prefix, making it an incomplete and invalid email domain for any user. The onmicrosoft.com root is not directly usable as an address space; each tenant owns a unique subdomain. Thus, the correct default address must include the tenant name.
- ✓
username@<tenantname>.onmicrosoft.com
Why this is correct
When the tenant is provisioned, Microsoft creates a unique initial domain in the format <tenantname>.onmicrosoft.com, which is automatically registered and verified. New users are assigned the user principal name and email address using this domain by default, because no custom domains have been added yet. This domain remains the default until a custom domain is added and set as primary. Therefore, username@<tenantname>.onmicrosoft.com is the correct email suffix after signing up for Microsoft 365 Business Standard.
- ✗
username@microsoftonline.com
Why it's wrong here
microsoftonline.com is not a valid mailbox domain; it is used exclusively for Azure AD and Microsoft 365 service authentication endpoints, such as login.microsoftonline.com. User email addresses must use either the tenant's initial onmicrosoft.com domain or a verified custom domain registered for the organization. Emails sent to username@microsoftonline.com would fail because no mailbox organization is associated with that domain. Hence, this option is not a possible default email address for a new user.
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Microsoft 365 Tenant Setup
Key term
Microsoft 365 tenant
A Microsoft 365 tenant is a dedicated, isolated instance of Microsoft 365 services created for an organization when they sign up for a subscription.
Key term
Microsoft 365
Microsoft 365 is a subscription-based cloud service from Microsoft that combines productivity tools like Office apps with security, device management, and online storage.
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