Security Policy Errors

Fix Microsoft 365 Emails Frequently Sent to Quarantine

John Wilson Oct 13, 2026 868 views

Question details

Users are experiencing issues where incoming or outgoing Microsoft 365 emails are frequently being flagged and sent to quarantine instead of being delivered to the inbox.

Why Are Microsoft 365 Emails Frequently Sent to Quarantine?

Product: Microsoft 365
Device & OS: not provided
Scenario: Troubleshooting email delivery failures and managing spam or malware filtering misclassifications in Microsoft 365.
Observed behavior: Legitimate messages are frequently sent to quarantine due to strict spam filtering, invalid recipient addresses, full mailboxes, or misconfigured mail-flow rules.

Before you start

Ensure you have administrator or security admin privileges in your Microsoft 365 tenant to access the Defender portal, review message traces, and modify anti-spam policies.

Solution 1Recommended

Review Quarantine Reasons and Run a Message Trace

Investigate specific quarantined emails to determine the exact root cause, such as spam filtering triggers, malware detection, or mail-flow rule blocks.

Before releasing any quarantined emails, it is crucial to understand why they were flagged. Releasing malicious emails blindly can compromise your organization's security.

Access the Defender Portal

Navigate to Quarantine

Go to 'Email & collaboration' in the left-hand navigation pane, select 'Review', and then click on 'Quarantine'.

Check the Quarantine Reason

Select a quarantined message from the list to open its details panel. Look at the 'Quarantine reason' to see if it was blocked by a spam policy, phish filter, or a specific mail-flow rule.

Run a Message Trace

Open the Exchange admin center, go to 'Mail flow', and select 'Message trace'. Run a trace on the affected sender or recipient to view deeper insights into the delivery failure and routing events.

Do not release blindly: Do not release messages solely because they are quarantined. Always verify the sender and the context of the email to ensure it is not a disguised phishing attempt.

Solution 2

Adjust Anti-Spam Policies and Safe Sender Configuration

Modify your tenant's allow lists to prevent legitimate senders from being incorrectly classified as spam in the future.

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Frequently Asked Questions

Can full recipient mailboxes cause emails to be quarantined?

Yes. When an email is sent to an invalid address or a full mailbox, the delivery fails. Depending on your organization's mail-flow rules or outbound spam policies, these non-delivery reports (NDRs) or failed messages might be routed to quarantine to prevent backscatter spam.

Is it safe to release all quarantined emails if I trust my network?

No. It is highly discouraged to release messages solely because they are quarantined. Microsoft 365's protection mechanisms catch malware and phishing attempts that bypass standard filters. Always review the quarantine reason and message headers first.

How do I stop internal emails from going to quarantine?

Internal emails usually shouldn't go to quarantine unless they trigger specific malware or high-confidence phishing filters. Review your Exchange mail-flow rules to ensure internal routing isn't mistakenly triggering anti-spam policies, and verify your domain's SPF/DKIM records are properly configured.

Why would legitimate newsletters keep going to quarantine?

Newsletters often trigger bulk email filters. If a legitimate newsletter is frequently quarantined, you can review your anti-spam policy in the Defender portal and adjust the Bulk Complaint Level (BCL) threshold, or add the specific sender to your Tenant Allow/Block List.

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