Send & Receive Issues

How to Fix Microsoft 365 High-Severity Alerts Blocking Email

Tauseeq Magsi Oct 10, 2026 871 views

Question details

Users are unable to send outbound messages due to a high-severity security or service alert triggered on their Microsoft 365 email account.

How to Fix a High-Severity Microsoft 365 Alert Blocking Your Emails

Product: Microsoft 365
Device & OS: not provided
Scenario: Attempting to send outbound emails when the account is blocked by a security alert.
Observed behavior: The email account is prevented from sending outbound messages, and a high-severity security or service alert is displayed requiring administrator intervention.

Before you start

Ensure you have Microsoft 365 Administrator privileges, as investigating and resolving high-severity alerts requires access to the Microsoft 365 Admin Center and Microsoft Defender.

Solution 1Recommended

Open a Service Request in the Microsoft 365 Admin Center

Since high-severity alerts require back-end investigation, submitting a service request allows Microsoft Support to collect service logs and unblock your account.

A high-severity alert indicates a significant security risk, such as a compromised account or bulk spam activity. Microsoft automatically blocks outbound mail to protect the network. Only Microsoft Support can fully investigate the back-end logs to clear these specific types of blocks.

Access the Admin Center

Navigate to Help & Support

Click on the 'Support' icon at the bottom right of the screen or select 'Help & support' from the left-hand navigation menu.

Provide Alert Details

Create a new service request. Clearly state that outbound email is blocked and provide the specific alert ID, the affected mailbox address, the exact time of the occurrence, and any accompanying error messages (such as an NDR code).

Submit the Request

Submit the ticket so the Microsoft Support team can collect the necessary service logs and investigate the alert from their end.

Open a Service Request in the Microsoft 365 Admin Center

Review Defender Documentation: While waiting for support, you can review relevant security alerts in the Microsoft Defender for Cloud documentation (Azure Defender) to better understand the nature of the block and secure the affected account.

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Frequently Asked Questions

Why did Microsoft 365 block my email account from sending messages?

Microsoft 365 typically blocks outbound emails if the system detects suspicious activity, such as sending bulk spam, signs of a compromised account, or triggering a high-severity security alert in Microsoft Defender.

Where can I find the alert ID for my blocked email?

The alert ID is usually included in the non-delivery report (NDR) bounce message you receive when an email fails to send, or it can be found in the Alerts section of the Microsoft Defender security portal.

Can a standard user unblock a Microsoft 365 email account?

No. Removing a high-severity block generally requires a Microsoft 365 administrator to review the alert in the Microsoft 365 Defender portal and, if necessary, submit a support ticket to Microsoft.

How long does it take for Microsoft to remove an email block?

Once the root cause is addressed and the service request is submitted, it typically takes a few hours for Microsoft Support to investigate the logs and unblock the affected mailbox, though complex security breaches may require additional time.

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