MS-102 Deploy and manage a Microsoft 365 tenant Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  Update the MX record in the DNS to point to Microsoft 365 with the custom domain.

  Why it's wrong here

  Updating the MX record in DNS directs inbound email flow to Microsoft 365 but does not influence which domain is assigned as the default for new users. The default domain setting controls the UPN and primary SMTP address for newly created user accounts, while MX only affects message routing for the custom domain's mailboxes. Even with a correct MX record, new users would still be created under the onmicrosoft.com domain unless you explicitly change the default in the admin center.

• ✓

  In the admin center, go to Settings > Domains, select the custom domain, and click 'Set as default'.

  Why this is correct

  This is the correct method. In the Microsoft 365 admin center, navigating to Settings > Domains, selecting the verified custom domain, and clicking 'Set as default' changes the tenant-level default domain. Once set, all new users are automatically assigned a user principal name (UPN) and primary SMTP address using that custom domain, rather than the initial onmicrosoft.com domain. This is a global, automated setting that applies to every subsequently created user, making it the intended administrative control.

• ✗

  Use the Exchange admin center to set the default email address policy to use the custom domain.

  Why it's wrong here

  The Exchange admin center (EAC) does not contain a setting to define the tenant's default domain for new user creation. While Exchange Online has email address policies that can add or modify email addresses on existing mailboxes, these policies operate at the mailbox level and do not control the tenant-wide default domain used for new users. Moreover, the default domain for sign-in and UPN is managed in the Microsoft 365 admin center, not through Exchange-specific policy configuration.

• ✗

  For each new user, manually add an email alias with the custom domain and remove the onmicrosoft.com alias.

  Why it's wrong here

  Manually adding an email alias with the custom domain and removing the onmicrosoft.com alias for each new user is operationally incorrect because it is a per-user, reactive workaround that does not scale or persist for future accounts. This process would need to be repeated for every user, whereas setting the custom domain as default automatically applies the desired domain to all new users. Additionally, removing the onmicrosoft.com alias from the mailbox does not change the user principal name, so sign-in might still default to the onmicrosoft.com domain unless separately changed, and doing this manually for every user is error-prone and inefficient.

Go deeper

Related to this question