Ho to Solve returning external emails

Question

Ho to Solve returning external emails

Tsholofelo Shong 0

How to solve the issue of Message-ID: [Moderator note: Personally Identifiable Information removed]@CT2P275MB0465.ZAFP275.PROD.OUTLOOK.COM to all our tenants for external mails

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Answer 1

Demi Nguyen 2,510 Independent Advisor

Hello Tsholofelo Shong,

Thank you for providing the details.

The Message-ID identifies the email but does not explain why it was returned. To better understand the delivery issue, could you please confirm the following?

Are emails being returned when sent to all external domains, or only to specific domains?
Are all users in the tenant affected, or only certain mailboxes?
Can users still send and receive internal emails within the organization?
What is the complete SMTP error code shown in the non-delivery report, or NDR?
Could you provide a screenshot or copy of the Diagnostic information section?

Before sharing the information, please hide all email addresses, the complete Message-ID, IP addresses, tenant details, and other organizational information.

In the meantime, if you have Exchange administrator access, please open the Exchange Admin Center and go to Mail flow > Message trace. Run a trace using an affected sender, external recipient, and the approximate time when the message was sent.

You can refer to the following Microsoft documentation: Message trace in the Exchange Admin Center | Microsoft Learn

The SMTP error code, diagnostic details, and message trace result are important because external messages can be returned for several different reasons. Please share these details so I can identify what is causing the bounce-back and provide more appropriate guidance.

Kind regards,

Tsholofelo Shong 0 Reputation points

2026-10-12T10:52:28.2+00:00

All external domains emails being returned - yes

All Users are affected

Internal emails we received - Yes

I havent resolve any problem
Tsholofelo Shong 0 Reputation points

2026-10-12T10:54:58.08+00:00

Message ID

<[Moderator note: Personally Identifiable Information removed]@JN1P275MB2399.ZAFP275.PROD.OUTLOOK.COM>

MessageTrace ID

[Moderator note: Personally Identifiable Information removed]

Message size‎104.39‎ KB

From IP [Moderator note: Personally Identifiable Information removed]

To IP
Demi Nguyen 2,510 Reputation points Independent Advisor

2026-10-12T12:38:41.8266667+00:00

Hi Tsholofelo Shong,

The Message ID and Message Trace ID identify the email, but they do not show the exact reason why the message was returned.

Could you please send a test email to an external recipient and then share a screenshot of the bounce-back message or Non-Delivery Report (NDR) you receive? I specifically need the complete SMTP error code, such as 4.x.x or 5.x.x, together with the Diagnostic information for administrators section.

A Non-Delivery Report normally includes an error code, the reason the message was rejected, and technical details that help identify whether the issue is related to outbound restrictions, spam protection, connectors, domain configuration, or the receiving mail server.

You can see examples of common NDR error codes in the following Microsoft documentation: Email nondelivery reports and SMTP errors in Exchange Online | Microsoft Learn

Before sharing the screenshot, please hide all email addresses, the full Message ID, IP addresses, tenant details, and other sensitive organizational information.

Once I have the complete NDR error code and diagnostic details, I can better understand why all external emails are being returned and suggest the appropriate next step.

Kind regards,
Demi Nguyen 2,510 Reputation points Independent Advisor

2026-10-13T14:25:29.94+00:00

Just checking in to see if you need any further assistance. If you have any follow-up questions or new updates, feel free to let me know.

Answer 2

The 550 5.7.233 NDR means the tenant has reached its Tenant External Recipient Rate Limit (TERRL), so external sending will resume automatically when the recipient count for the rolling 24-hour window falls below the tenant’s limit.^1^

The Message-ID ending in PROD.OUTLOOK.COM is only the message identifier; it is not the rejection reason.

Restore external mail flow

Pause nonessential external sending across the tenant.
TERRL applies to the entire organization, so moving the send to another mailbox does not bypass it. Distribution-group members are counted individually after expansion.^2^
Wait for the rolling 24-hour count to decrease.
This is not necessarily a fixed midnight reset. Each recipient drops from the count 24 hours after the relevant message was sent. Once usage is below the limit, users can send externally again.^1^
Check current consumption and the tenant limit.
In the Exchange admin center, go to: Reports → Mail flow → Tenant Outbound External Recipients.
This report shows the tenant’s TERRL and usage.^3^
After usage falls below the limit, send one external test message.
Confirm that it is delivered rather than generating another 5.7.233 NDR.

TERRL scales automatically with the number of licenses in the organization. It is not an Outlook Classic setting, so reinstalling Outlook, recreating profiles, or changing client settings will not resolve this tenant-wide block.^2^

Identify what consumed the allowance

If the volume was unexpected:

Use Exchange admin center → Mail flow → Message trace to identify the mailboxes or applications responsible.
In the Microsoft Defender portal, review the built-in alerts:
- Email sending limit exceeded
- Suspicious email sending patterns detected
- User restricted from sending email
Investigate any unexpected sender activity as a possible compromised account before restoring normal sending.

These monitoring and alert options are documented in Microsoft’s outbound-limit troubleshooting guidance.^2^

Prevent recurrence

Spread legitimate external sends over time.
Remember that a group containing 1,000 external members consumes 1,000 external-recipient units.
Create a custom alert at a suitable threshold—for example, 80% of TERRL—so administrators can reduce or redirect sending before the tenant is blocked.
For bulk, marketing, billing, or other high-volume external mail, use Azure Communication Services Email or a dedicated bulk-email provider. Exchange Online is not designed as a bulk-mail platform.^2^

If the EAC report shows usage below the displayed TERRL but new messages still return 550 5.7.233, open a Microsoft 365 support request and include the full NDR, UTC timestamp, sender, recipient domain, and a screenshot/export of the Tenant Outbound External Recipients report.

References