Security Policy Errors

How to Fix Microsoft 365 Accounts Repeatedly Marked as Bad Outbound Senders

Maira Mehtab Sep 21, 2026 869 views

Question details

Business-development accounts sending moderate daily email volumes are continually flagged and blocked as bad outbound senders in Microsoft 365, even after being re-enabled.

Product: Microsoft 365
Device & OS: not provided
Scenario: Sending outbound business-development emails (200-300 daily) through Microsoft 365.
Observed behavior: Outgoing mail is blocked as potentially compromised, and the accounts are repeatedly marked as bad outbound senders despite administrative unblocking.

Before you start

Ensure you have global or security administrator access to the Microsoft 365 Defender portal to review restricted entities and modify security configurations.

Solution 1Recommended

Unblock the Account and Enforce Security Measures

First, verify the account is not compromised, enforce Multi-Factor Authentication (MFA), and safely unblock the user from the restricted entities page.

Microsoft 365 will automatically disable sending capabilities for accounts that exhibit suspicious behavior or exceed spam thresholds to protect your domain's reputation. Re-enabling the account without addressing underlying vulnerabilities often results in an immediate re-block.

Reset user credentials

Go to the Microsoft 365 admin center, locate the affected user account, and reset their password immediately to sever any potential unauthorized access.

Enable MFA

Ensure Multi-Factor Authentication is enforced for the affected account to prevent future compromises.

Access Microsoft 365 Defender

Unblock the account

Select the blocked account from the list, choose the option to unblock it, and follow the on-screen prompts to restore sending privileges.

Solution 2

Audit Email Authentication and Sending Patterns

Ensure your domain's DNS records are properly configured and audit your mailing lists to prevent legitimate emails from being flagged as spam.

Solution 3

Escalate to Microsoft Support

If the issue persists despite correct configurations and clean sending patterns, contact Microsoft directly for log analysis.

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Frequently Asked Questions

Why does Microsoft 365 flag an account as a bad outbound sender?

Accounts are flagged when they exceed Microsoft's sending limits, trigger internal spam filters due to poor email content or list quality, or exhibit login behavior that indicates a potential account compromise.

How long does it take to unblock a restricted entity in Microsoft 365?

After successfully unblocking an account in the Microsoft 365 Defender portal's Restricted entities page, it typically takes up to an hour for the system changes to propagate and allow the user to send emails again.

Do I need SPF, DKIM, and DMARC for a Microsoft 365 account?

Yes, configuring SPF, DKIM, and DMARC is crucial for establishing domain reputation and ensuring email deliverability. Without these authentication protocols, your outgoing emails are highly likely to be flagged as spam or blocked entirely by Microsoft and external providers.

Can third-party email tools cause outbound sender blocks?

Yes, using third-party marketing tools, automation scripts, or CRM integrations to send bulk emails through Microsoft 365 without proper API throttling or domain authentication can trigger automated outbound sender restrictions.

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