Fix Undeliverable Messages from External Users in Microsoft 365
Question details
External senders are receiving nondelivery reports when attempting to email internal Microsoft 365 users due to messages queuing during security checks.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- External users attempt to send emails to an internal Microsoft 365 organization but fail to reach the recipient.
- Observed behavior
- Messages remain queued in the background for security checks or sandbox processing, eventually timing out and generating a postmaster nondelivery report (NDR) back to the sender.
Ensure you have global administrator privileges in your Microsoft 365 tenant, as resolving backend routing and security queue issues requires access to the admin center.
Open a Service Request in the Microsoft 365 Admin Center
Because this issue affects backend message-delivery logs, a Microsoft 365 global administrator must request intervention from Microsoft Support to review message traces and backend logs.
When emails get stuck in Microsoft 365 sandbox processing (such as Microsoft Defender for Office 365 Safe Attachments scanning), the standard admin tools may not provide enough control to unblock the queue. Microsoft Support engineers have the necessary backend access to investigate and clear these specific queues.
Sign in to the Microsoft 365 admin center (admin.microsoft.com) using a global administrator account.
Click on the 'Support' icon in the lower-right corner or select 'Help & Support' from the left-hand navigation menu.
Type a brief description of the issue (e.g., 'External emails generating NDRs due to security sandbox queuing') and press Enter. Click on 'Contact Support' to open a new ticket.
Include example sender and recipient email addresses, timestamps, and a copy of the Non-Delivery Report (NDR) headers to help the support team expedite the investigation.
Equip Your Team with WPS Office for Seamless Productivity
While your administrators resolve backend Exchange and email routing issues, ensure your team's document workflows remain uninterrupted. WPS Office is a powerful, lightweight alternative to Microsoft Office that is completely free for basic use.
- 1. Download the Installer: Visit the official WPS Office website and click the free download button for your operating system.
- 2. Install the Software: Run the downloaded installer file and follow the on-screen instructions to complete the setup.
- 3. Open and Work: Launch WPS Office to instantly create, open, and edit your existing Microsoft Office files without formatting loss.
Frequently Asked Questions
What causes an email to get stuck in sandbox processing in Microsoft 365?
Features like 'Safe Attachments' and 'Safe Links' in Microsoft Defender for Office 365 scan incoming emails by opening attachments in a virtual environment (sandbox). If the Microsoft backend experiences processing delays, the email is queued, which can eventually trigger an NDR.
Can I use Message Trace to investigate this issue myself?
Yes. A global or Exchange administrator can run a Message Trace in the Exchange admin center to verify where the email stopped. However, if the trace shows the message is stuck in a 'processing' or 'filtering' state indefinitely, Microsoft Support intervention is still required.
What information should I collect from external senders?
Ask the external sender to provide the complete Postmaster Non-Delivery Report (NDR) email. The NDR contains vital diagnostic headers, error codes, and server routing information that Microsoft Support needs to trace the failure.
