MS-102 Deploy and manage a Microsoft 365 tenant Practice Question
A company recently added the custom domain 'contoso.com' to their Microsoft 365 tenant. Users report that they cannot receive external email sent to their new domain addresses. The administrator confirmed that the domain status shows 'Active' in the Microsoft 365 admin center. What is the most likely cause of this issue?
⚠ Common exam trap
The trap here is that candidates see 'Active' domain status and assume all DNS configurations are correct, but Microsoft 365 separates domain verification (TXT record) from mail routing (MX record), so a verified domain can be 'Active' yet still unreachable for inbound email if the MX record is misconfigured.
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
✓
The MX record for the domain is missing or points to an incorrect mail server.
The domain status 'Active' in the Microsoft 365 admin center indicates that the domain has been successfully verified and added to the tenant. However, for external email to be delivered to users at that domain, the public MX record in DNS must point to Microsoft 365's mail servers (e.g., contoso-com.mail.protection.outlook.com). If the MX record is missing or points to an incorrect server, external senders cannot route email to the tenant, even though the domain is verified and active.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The domain was not verified with a TXT record.
Why it's wrong here
Domain verification via a TXT record proves DNS ownership and is a one-time administrative requirement before using the domain in Microsoft 365. However, verification does not create an email routing path, and the domain status showing 'Active' means this step was already completed. Since a missing TXT record would only prevent initial verification, it cannot explain why external email is not being received. The actual cause must be the MX record, which is solely responsible for directing inbound SMTP traffic.
- ✓
The MX record for the domain is missing or points to an incorrect mail server.
Why this is correct
The MX record is the authoritative DNS resource that specifies the mail exchanger for a domain. When an external sender tries to deliver to contoso.com, their mail server queries DNS for the MX record to discover which host accepts inbound messages. If this record is absent or points to an incorrect mail server, delivery to Exchange Online cannot occur, causing non-delivery reports or messages routed to the wrong destination. Microsoft 365 requires the MX record to point to contoso-com.mail.protection.outlook.com with the correct priority.
- ✗
The SPF record for the domain is missing or incorrectly configured.
Why it's wrong here
An SPF record is a TXT record that lists the servers authorized to send email for the domain. Receiving mail systems use it to authenticate outbound messages and prevent spoofing, so SPF has no bearing on whether your own mailbox can accept incoming mail. Inbound delivery relies exclusively on the MX record to advertise your mail server, not on SPF. Therefore, a missing or malformed SPF record will not block external messages from being delivered to Exchange Online.
- ✗
The custom domain was not added to the user's primary email address.
Why it's wrong here
The user's primary SMTP address determines the default From address and how replies are routed, but it does not control inbound delivery. If the verified custom domain is enabled as an accepted domain and the user has a contoso.com proxy address, messages to that address will be delivered as long as the MX record is correct. Conversely, changing the primary address to contoso.com cannot compensate for a missing MX record, because external senders would still have no way to locate your mail server and initiate delivery.
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Microsoft 365 Tenant Setup
Key term
Custom domain
A custom domain is a personalized internet address (like contoso.com) that you can use with cloud services instead of the default domain provided by the service provider.
Key term
Outlook
Microsoft Outlook is an email, calendar, and contact management application that is part of the Microsoft 365 suite, used by businesses and individuals to organize communications and schedules.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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