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How to Fix Microsoft 365 NDR 550 5.7.703 Domain Block
Question details
Users are experiencing email delivery failures when sending messages to specific domains, receiving a Non-Delivery Report (NDR) indicating a block.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Sending emails to external addresses such as .in domains.
- Observed behavior
- Messages are rejected with NDR 550 5.7.703 because the Microsoft 365 tenant's Tenant Allow/Block List is actively blocking the destination domain.
Ensure you have global administrator or security administrator permissions in your Microsoft 365 tenant to access and modify the Microsoft Defender portal settings.
Modify the Tenant Allow/Block List in Microsoft Defender
Remove incorrect domain blocks or add an allow entry in the Microsoft Defender portal to restore outbound email flow to the affected domains.
The NDR 550 5.7.703 error strictly indicates that your own organization's security policies are preventing the email from leaving your tenant. Adjusting the Threat Policies resolves this restriction.
Log in to the Microsoft Defender portal using your administrator credentials and navigate to the 'Email & collaboration' section on the left sidebar.
Click on 'Policies & rules' from the menu, then select 'Threat policies'.
Under the Rules section, click on 'Tenant Allow/Block List'. Navigate to the 'Domains & addresses' tab to review the current entries.
Search for the blocked domain (e.g., circumference.in). If an incorrect block exists, select it and click the delete or remove icon. Alternatively, add an appropriate allow entry for the domain if required.
Allow some time for the changes to propagate across the Exchange Online Protection nodes. Also, check your mail-flow rules in the Exchange admin center to ensure no conflicting rules exist.
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Frequently Asked Questions
What does NDR 550 5.7.703 mean in Microsoft 365?
NDR 550 5.7.703 is a bounce-back error indicating that an outbound email was rejected because the destination domain is explicitly blocked by your organization's Tenant Allow/Block List in Microsoft 365.
How long does it take for Tenant Allow/Block List changes to work?
Most modifications to the Tenant Allow/Block List propagate within 5 to 30 minutes, but Microsoft officially states it can take up to 24 hours to sync across all worldwide Exchange Online Protection servers.
Can mail flow rules override the Tenant Allow/Block List?
Mail flow (transport) rules are evaluated alongside threat policies. However, a strict block in the Tenant Allow/Block List typically takes precedence for security reasons. Administrators should review both the Defender portal and the Exchange admin center to ensure alignment.
