MX record host unresolvable

Kelley Arnold 0 Reputation points
2026-09-09T22:46:26.31+00:00

Some of my incoming emails are not being delivered, or are delayed. Error says MX record host unresolvable. How do I fix this?

Outlook | Web | Outlook on the web for business | Email
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  1. AI answer

    2026-09-09T22:46:44.15+00:00

    MX record host unresolvable usually means the domain’s DNS MX record is missing, incorrect, stale, or points to a host name that DNS cannot resolve. That can cause inbound mail to fail or be delayed.

    To fix it:

    1. In the Microsoft 365 admin center, go to Settings > Domains and select the affected domain.
    2. Open Required DNS settings and select Check DNS.
    3. Verify the MX configuration for the domain.
      • There should be only one MX record for a domain enrolled in Exchange Online.
      • For Exchange Online, the MX record should point to the Microsoft 365 mail host for the domain, such as <your domain>.mail.protection.outlook.com or <your domain>.subdomain.mx.microsoft.
    4. If Microsoft 365 reports DNS problems, follow the recommended fix.
      • If Microsoft hosts the DNS, the change might be available directly in the admin center.
      • Otherwise, update the MX record at the domain registrar or DNS hosting provider.
    5. Use nslookup to confirm the MX record exists and resolves correctly in DNS.
    6. Test mail flow by running the Inbound SMTP Email and Outbound SMTP Email tests in the Microsoft Remote Connectivity Analyzer.

    Important checks:

    • Remove old or conflicting MX records. Microsoft does not support multiple MX records for Exchange Online domains.
    • If mail fails only for one destination domain, the issue can also be on that destination domain’s side, including missing or incorrect MX records there.
    • DNS changes usually propagate in a few hours, but can take up to 72 hours.

    If the domain was recently added to Microsoft 365, also verify the domain shows as Healthy in Settings > Domains.

    References:

    AI-generated content may be incorrect. Read our transparency notes for more information.

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  2. Chris-D 1,180 Reputation points Independent Advisor
    2026-09-09T23:02:21.06+00:00

    Hi Kelley,

    Thank you for sharing the details of your situation.

    The error**“MX record host unresolvable”** generally means that the sending mail server cannot resolve or connect to the mail server hostname specified in your domain’s MX record. As a result, incoming messages may be delayed while the sender’s server retries delivery, or they may eventually be returned to the sender as undeliverable.

    Since the MX record is managed through your organization’s DNS configuration, this issue is not typically caused by Outlook on the web itself. Please contact your organization’s IT administrator or the administrator who manages your domain and DNS records, and share the following troubleshooting steps with them:

    1. Sign in to the Microsoft 365 admin center and go to Settings > Domains.
    2. Select the affected custom domain and review the MX record required by Microsoft 365.
    3. User's image
    4. Sign in to the domain’s DNS hosting provider or registrar and confirm that the existing MX record exactly matches the value shown in the Microsoft 365 admin center.
    5. Check whether the MX record’s destination hostname can be resolved through public DNS.
    6. Remove any outdated or invalid MX records that are no longer in use. However, do not remove records associated with an active third-party email security service, mail gateway, or hybrid Exchange environment without first verifying the mail-flow design.
    7. Run the domain or DNS health check from the Microsoft 365 admin center after correcting the record.
    8. Allow time for the DNS changes to propagate, and then test incoming email delivery again.
    9. If the issue affects only certain senders, ask one of them to share the complete nondelivery report, including the error code and diagnostic details, so your IT administrator can investigate further.

    If all mailboxes are hosted in Exchange Online and no third-party mail gateway is being used, the MX record normally points to a hostname ending in:

    .mail.protection.outlook.com

    However, your administrator should use the exact MX value displayed for your domain in the Microsoft 365 admin center rather than manually constructing it.

    For reference, you may also want to share these Microsoft articles with them:

    In case you do not know who is your IT admin, kindly refer to this article: How do I find my Microsoft 365 admin? - Microsoft Support  

    Additionally, if needed, your IT team can also raise a support ticket with Microsoft Support through the Microsoft 365 Admin Center for deeper investigation. They have access to backend configurations and can perform a more in-depth investigation. At the very least, they can provide the most effective workaround to ensure your experience remains smooth and secure.      

    I hope this information is helpful. Should you have any further questions or need additional assistance, please feel free to share them in the comment below. I'm very happy to help.  

    If the answer is helpful, please click 'Yes' and kindly upvote it.

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