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How to Troubleshoot Microsoft 365 Outbound Mail Rejections
Question details
The user is experiencing intermittent sender verification and domain reputation errors that prevent outbound emails from being delivered in Microsoft 365.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Sending outbound emails from a Microsoft 365 account to external recipients.
- Observed behavior
- Outbound emails bounce back or are rejected due to intermittent sender verification failures, domain reputation errors, recipient filtering, or tenant configuration issues.
Gather any Non-Delivery Reports (NDRs) or bounce-back messages you have received, as these contain crucial error codes, recipient addresses, and message headers required for effective troubleshooting.
Open a Support Case with Microsoft 365
The most effective way to resolve complex domain reputation and tenant configuration rejections is to provide your email diagnostics directly to Microsoft Support.
Because outbound mail rejections often stem from server-side configurations, IP reputation blocks, or recipient-side filtering, standard user-level troubleshooting is rarely enough. Microsoft's support team can analyze your message headers to pinpoint the exact failure point in the mail flow.
Save the complete Non-Delivery Reports (NDRs), message headers, recipient email addresses, and the exact timestamps of the failed deliveries.
Log in to the Microsoft 365 admin center using your administrator credentials.
Navigate to 'Support' and then 'Help & support'. Describe your outbound mail rejection issue, attach the collected NDRs and message headers, and submit the request.
If you do not have admin access, open your Outlook application, go to the 'Help' tab on the ribbon, and select 'Contact Support' to report the issue to the support team.
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- 1. Visit the website: Go to the official WPS Office website in your browser.
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- 3. Install and launch: Run the installer, follow the on-screen prompts, and open WPS Office to immediately begin creating and editing your documents.
Frequently Asked Questions
What is a Non-Delivery Report (NDR) in Microsoft 365?
An NDR is an automated bounce message sent by the mail server explaining why an email was not delivered. It contains an error code (such as 5.1.1 or 5.7.1), the reason for the rejection, and technical details needed by administrators to fix the issue.
Why is my domain reputation causing outbound emails to bounce?
If your IP address or domain has been flagged for sending spam, or if your DNS authentication records (like SPF, DKIM, and DMARC) are misconfigured, recipient servers may block your emails to protect their users and network infrastructure.
Can recipient filtering cause my outgoing emails to be rejected?
Yes. The recipient's organization may have strict anti-spam filters, custom mail flow rules, or specific safe sender lists that inadvertently block your outgoing messages based on their internal security policies.
How can I check my message headers for delivery errors?
You can use the Microsoft Remote Connectivity Analyzer's Message Header Analyzer tool. Simply copy and paste the headers from your NDR or bounced email into the tool to view detailed hop-by-hop delivery times, spam confidence levels (SCL), and specific failure points.
