Outlook unable to receive external emails

Question

Outlook unable to receive external emails

Brown Hannah 0

I am currently using MS365 Business Standard.

All emails sent from external sources are not received.

I changed the domain DNS settings to records provided by Microsoft 48 hours ago, and confirmed that the changes were all successful.

However, since then, emails are sent but not received.

On the sender, a message stating that the email could not be sent appears as shown below.

[gmail]

DNS Error: DNS type 'mx' lookup of [Operator's Note: Personal information has been deleted. responded with code NOERROR DNS type 'aaaa' lookup of [Operator's Note: Personal information has been deleted.]. responded with code NXDOMAIN DNS type 'a' lookup of [Operator's Note: Personal information has been deleted.]. responded with code NXDOMAIN

[naver]

421 4.4.1 Sorry, I wasn't able to establish an SMTP connection.

Please check this as soon as possible.

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2 answers

Your answer

Answer 1

Dear @Brown Hannah,

The error messages returned by Gmail (NXDOMAIN) and Naver (421 4.4.1 SMTP connection failed) indicate that external email servers cannot find a valid MX (Mail Exchanger) record pointing to Exchange Online for your domain. This usually occurs if the MX record was omitted, entered incorrectly, or left alongside conflicting old DNS records.

I recommend checking and resolving the issue through the following steps:

Step 1: Verify and Fix DNS Records in Microsoft 365 Admin Center

Go to the Microsoft 365 admin center.
Navigate to Settings > Domains.
Select your custom domain and click Check DNS (or Fix issues).
Look at the expected MX Record values under the Exchange Online section:
- Host Name: @ (or leave blank depending on your DNS host)
- Points to / Value: <your-domain-prefix>.mail.protection.outlook.com
- Priority: 0 (or 1)

Step 2: Update Your DNS Hosting Provider

Log into your DNS host (e.g., GoDaddy, Namecheap, Cloudflare) and verify the following:

Remove Old MX Records: Delete any previous MX records from your former email host. Having multiple conflicting MX records causes external senders to deliver mail to non-existent servers. Please refer to this document: External Domain Name System records for Microsoft 365
Correct Syntax Errors: Ensure there are no typos, extra trailing periods, or unnecessary characters in the Points to address.
Verify Record Type: Ensure the record type is explicitly set as MX, not an A or CNAME record.

Step 3: Test Propagation

Once saved, test your MX record publicly using a free tool such as MxToolbox. If it successfully resolves to .mail.protection.outlook.com, external mail flow will resume immediately.

MxToolbox link is shared by community members for your convenience. It points to a third-party site that is not managed or verified by Microsoft. I can’t guarantee the quality, safety, or suitability of any content or software found there. Please review carefully and make sure you understand any potential risks before using it.
For more details on external DNS record requirements, please refer to the official Microsoft DNS Guidance Document.

Please feel free to reply if the issue persists after updating your provider's settings.

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BabybooHN 2,955 Reputation points Independent Advisor

2026-08-12T01:24:42.4966667+00:00

Dear @Brown Hannah,

I hope you’re doing well. I just wanted to check in and see how things are going with the steps I shared earlier. If you ran into any difficulties at any point or if something is unclear, please don’t hesitate to let me know. I completely understand that these processes can sometimes be tricky, and I’m here to make it as easy as possible for you.

Feel free to share any details or questions you might have, I’m ready to listen and assist you further.

Answer 2

The sender errors indicate an inbound DNS routing problem for the custom domain, specifically that the domain is not resolving to a valid mail destination. The mx lookup returning no usable destination, together with a and aaaa lookup failures, matches a missing or incorrect MX record scenario.

Recommended checks:

In the Microsoft 365 admin center, go to Setup > Domains and check the domain Status. If there is an issue, open Check health.
Confirm the domain is verified in Microsoft 365.
In the domain’s DNS Settings for Exchange Online, confirm the MX record exactly matches the value shown there.
Use nslookup.exe to verify that the MX record exists publicly for the domain.
At the DNS host, remove or correct any old or conflicting MX records and make sure the Microsoft 365 MX record is present.
If the external DNS zone is hosted by a third party, make the correction there or contact that provider.
Allow for DNS propagation. Microsoft documents that MX replication across DNS servers can take up to 72 hours.

If the MX record must continue pointing somewhere else, mail will not automatically arrive in Microsoft 365. In that case, use one of these supported options:

Set up a hybrid deployment.
Set up forwarding to the Microsoft 365 users’ Online Email Routing Address (domain.onmicrosoft.com).

Based on the reported Gmail and Naver errors, the highest-probability cause is that the public MX record for the domain is still missing, incorrect, or not yet fully propagated.