Outbound Emails to External Recipients Not Delivered – No NDR or Bounce-Back

Question

Outbound Emails to External Recipients Not Delivered – No NDR or Bounce-Back

JC Lopez 0

Hello Community,
I’m experiencing an issue with Microsoft 365 email and need assistance.

Issue:
I can reply to emails from external recipients without any problem. - However, when I send a new email to an external recipient, they never receive it. - There are no Non-Delivery Reports (NDR) or bounce-back messages. - The email shows as “Sent” in Outlook Web App (OWA) and desktop client. - Internal emails work fine.
Troubleshooting Done: - Tested sending from OWA and Outlook desktop – same result. - IT confirmed message trace and outbound spam policy checks. - No errors or alerts on my end.

Impact: - Business-critical communication is blocked.

Has anyone encountered this issue before? What steps can Microsoft take to resolve this? Could this be related to outbound restrictions or tenant-level policies? Any guidance or escalation advice would be greatly appreciated. Thank you!

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2 answers

Your answer

Answer 1

Kudos-Ng 14,985 Microsoft External Staff Moderator

Hi JC Lopez,

Thank you for posting your question in the Microsoft Q&A forum.

From your description, it appears you’ve already completed several troubleshooting steps and narrowed the issue specifically to new emails sent to external recipients (while replies to the same recipients work), and you’ve reproduced the behavior across both Outlook on the web (OWA) and the Outlook desktop client.

I performed some research but have not found any recent, confirmed reports that exactly match your scenario. To better understand and help isolate the cause, could you please share the following details:

Is this affecting multiple users in your tenant, or is it limited to just your account?
Which external recipient providers have you tried (e.g., Microsoft/Outlook.com, Gmail, Yahoo, corporate domains behind third‑party gateways)?
Did this start following any recent Outlook or service update, or did it occur suddenly without a known change?
Have you tested sending from a different network (e.g., mobile hotspot, home/office Wi‑Fi, VPN on/off) to rule out any network or egress filtering issues?

Looking forward to your input.

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JC Lopez 0 Reputation points

2025-12-11T20:24:32.55+00:00
Hi Dan,

Appreciate your reply. Please see my answers below.

Is this affecting multiple users in your tenant, or is it limited to just your account?

This is just affecting my email.

Just to add: We have had a breach on one of my colleagues email a couple of weeks ago where his email sent what we believe is a phishing email that leads to an external link. That seemed to block most of our outgoing mail for 2 weeks.

Now, everything seems to be working with my workmates apart for my email.

Which external recipient providers have you tried (e.g., Microsoft/Outlook.com, Gmail, Yahoo, corporate domains behind third‑party gateways)?

Mailing to my personal email (gmail) works.

Extrnal company emails doesn't.

Did this start following any recent Outlook or service update, or did it occur suddenly without a known change?

I am only 1month in the company so my email is new. I'm not sure if my email has been set up properly. IT suspects that the issue needs to be sorted out with Microsoft.

Have you tested sending from a different network (e.g., mobile hotspot, home/office Wi‑Fi, VPN on/off) to rule out any network or egress filtering issues?

Unfortunately no, but I will try today and get back to you asap.

Thank you for your assistance!
Kudos-Ng 14,985 Reputation points Microsoft External Staff Moderator

2025-12-12T08:46:59.2933333+00:00
Hi JC Lopez,

Thank you for the detailed update and clear additional context.

Given your note that other mailboxes in your organization were affected by outbound restrictions after a colleague’s account breach, it’s possible your account still carries residual risk signals. In practice, personal mailbox (e.g., Gmail) may accept your messages, while external organizations with stricter security gateways can silently block them before delivery.

To troubleshoot further, could you coordinate with the recipient side as follows?

Ask the external company’s IT/admin to check their message trace to confirm whether your emails actually enter their environment (i.e., arrive at their Microsoft 365/EOP or upstream email gateway). If the message never appears in their trace, it likely was blocked upstream (e.g., at their gateway/edge).

Ask one recipient to add your address to Outlook’s Safe Senders list, then try sending a new test email. Guidance: https://support.microsoft.com/en-us/office/add-recipients-to-the-safe-senders-list-in-outlook-be1baea0-beab-4a30-b968-9004332336ce#picktab=new_outlook

Note: Safe Senders helps once a message reaches the recipient’s mailbox; it won’t override any blocks at the recipient’s gateway/edge, but it’s useful to verify mailbox-level delivery once the message is inside their tenant.

Hope this help you in troubleshooting this issue further.
JC Lopez 0 Reputation points

2025-12-14T19:48:29.12+00:00

Hi,

Issue on my colleague's mail has been resolved the same week the breach was discovered.
I have also spent an hour with our IT (another person) and he found out that it was a hyperlink in my signature that was causing the issue.
Everything is now back to normal after we have deleted the hyperlinks.

Thanks for your time, and appreciate the help.
Kudos-Ng 14,985 Reputation points Microsoft External Staff Moderator

2025-12-18T06:57:39.0933333+00:00

Hi JC Lopez,

Thank you so much for your kind response. I truly appreciate it!

I’m really glad I could support you, and I believe the updates and insights you’ve shared will be valuable not only for the community but also for myself to continue improving and learning.

Thank you again for your engagement and support.

Warm regards,

Answer 2

It sounds like you are experiencing a frustrating issue with sending outbound emails to external recipients in Microsoft 365. Here are some potential causes and troubleshooting steps you can consider:

Check for Throttling: If suspicious activity is detected from your IP address, Microsoft 365 may temporarily restrict mail from that source. You might receive a Non-Delivery Report (NDR) indicating this, but in some cases, it may not be generated. You can check for throttling messages by looking for specific error codes or contacting Microsoft support for assistance.
Review Outbound Spam Policies: Since your IT team has already checked outbound spam policies, ensure that there are no rules that might be blocking or filtering your outgoing emails to external addresses. Sometimes, policies can inadvertently affect email delivery.
Message Trace: Since you mentioned that message trace was conducted, ensure that the trace includes a detailed search for the specific time frame and recipient addresses. This can help identify if the messages are being processed correctly or if they are being dropped due to filtering.
Distribution Group Settings: If you are sending emails to a distribution group that includes external recipients, ensure that the ReportToManagerEnabled and ReportToOriginatorEnabled parameters are set correctly. If both are set to False, it may prevent delivery reports from being sent back to you.
Check for Connectivity Issues: Ensure that your network allows connections to Microsoft 365 datacenters. Sometimes, firewall settings can block outbound connections, preventing emails from being delivered.

If these steps do not resolve the issue, consider escalating the matter to Microsoft support for further investigation, as they can provide more in-depth analysis and assistance.

References: