Security Policy Errors

Fix Microsoft Defender Outbound Anti-Spam Policies for Shared Mailboxes

Maira Mehtab Oct 9, 2026 868 views

Question details

A custom, higher-priority Microsoft Defender outbound anti-spam policy is not applying when sending bulk emails from a shared mailbox.

How to Fix Microsoft Defender Outbound Anti-Spam Policies Not Applying to Shared Mailboxes

Product: Microsoft 365 Defender
Device & OS: not provided
Scenario: An administrator has configured an outbound anti-spam policy to increase sending limits for a shared mailbox to allow bulk email delivery.
Observed behavior: The default outbound anti-spam policy continues to enforce lower limits and blocks the emails, ignoring the newly created higher-priority policy.

Before you start

Ensure you have Exchange Administrator, Security Administrator, or Global Administrator privileges in your Microsoft 365 tenant to view and modify Defender threat policies.

Solution 1Recommended

Verify Policy Scope, Priority, and Sender Conditions

Ensure that the custom policy is correctly targeted at the shared mailbox and check that its priority level takes precedence over the default tenant policy.

In Microsoft 365 Defender, custom policies must be explicitly assigned and given a higher priority (lower numeric value) than the default policy to take effect. Shared mailboxes require exact matching in the recipient/sender condition fields.

Access Defender Policies

Log in to the Microsoft 365 Defender portal at security.microsoft.com and navigate to Email & collaboration > Policies & rules > Threat policies > Anti-spam.

Edit the Custom Policy

Select your custom outbound anti-spam policy from the list and click Edit protection settings or Edit applied to depending on what needs adjusting.

Confirm Shared Mailbox Scope

Check the Applied to section to ensure the specific email address of the shared mailbox is explicitly included as a sender. Group memberships sometimes fail to apply correctly to shared mailboxes.

Check Policy Priority

Verify that the custom policy's priority is set to a higher priority (e.g., 0 or 1) so that it triggers before the Default outbound anti-spam policy.

Verify Policy Scope, Priority, and Sender Conditions

Policy Propagation Time: Changes to anti-spam policies may take up to 1 hour to fully propagate across the Microsoft 365 environment. Please wait before testing the email limits again.

Solution 2

Escalate to Microsoft Support or Community Q&A

If the policy scope and priority are correct but the default policy still applies, you may need to escalate the issue for advanced diagnostics.

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Frequently Asked Questions

Why is my shared mailbox restricted from sending emails?

Microsoft 365 enforces outbound spam limits to protect tenant reputation. If a shared mailbox exceeds these thresholds by sending too many bulk emails, it is automatically restricted by the default outbound anti-spam policy.

Can I completely disable outbound spam filtering in Microsoft 365?

No, Microsoft does not allow administrators to completely disable outbound anti-spam filtering. However, you can configure custom policies with higher thresholds for specific users or shared mailboxes.

How long does it take for a new outbound anti-spam policy to take effect?

After creating or modifying a threat policy in Microsoft Defender, it typically takes up to 1 hour for the changes to replicate and apply to all Exchange Online servers.

How do I unblock a shared mailbox that was blocked for sending spam?

An administrator must log in to the Microsoft 365 Defender portal, navigate to the Restricted entities page under Email & collaboration, select the blocked shared mailbox, and click Unblock.

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