Exchange Connection Problems

Why Exchange Online Blocks Messages as Spam and How to Fix It

WPS Editor Sep 28, 2026 869 views

Question details

Investigate and resolve issues where emails routed from an on-premises Exchange server to Microsoft 365 are incorrectly blocked or categorized as spam.

Why Exchange Online Blocks Messages as Spam and How to Fix It

Product: Microsoft Exchange Online
Device & OS: not provided
Scenario: Routing outbound messages from an on-premises Exchange server environment to a Microsoft 365 tenant.
Observed behavior: Messages are classified as spam, or the entire tenant is blocked by Exchange Online Protection (EOP).

Before you start

Ensure you have Microsoft 365 global administrator or Exchange administrator privileges to access the Exchange admin center and perform a message trace.

Solution 1Recommended

Run a Message Trace in the Exchange Admin Center

Use the built-in Message Trace utility in Exchange Online to determine exactly why emails are being flagged as spam or blocked.

Running a message trace allows administrators to track the journey of an email through the Exchange Online server. It reveals if a message was rejected, deferred, or delivered, and details the specific transport rules or spam classifications that acted upon it.

Sign in to the Exchange Admin Center

Navigate to the Exchange admin center (https://admin.exchange.microsoft.com) and log in using your Microsoft 365 global administrator or Exchange administrator account.

Access the Message Trace Tool

In the left-hand navigation pane, click on 'Mail flow' and then select 'Message trace' from the drop-down options.

Start a New Trace

Click the 'Start a trace' button. In the window that appears, enter the sender's or recipient's email address in the respective fields to filter the search.

Review Spam and Block Details

Execute the search and click on the specific blocked message from the results. Review the detailed delivery status, spam confidence level (SCL), and any transport rule triggers that caused the block.

Run a Message Trace in the Exchange Admin Center

Protecting Privacy: If you need to escalate this issue to Microsoft Support or share the trace report in a forum, be sure to redact or remove any sensitive private information (like full email addresses or user names) from the logs before sharing.

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Frequently Asked Questions

What is Exchange Online Protection (EOP)?

Exchange Online Protection is Microsoft's cloud-based email filtering service designed to protect organizations against spam, malware, phishing, and other inbound or outbound email threats.

Why would Exchange Online suddenly block my on-premises server?

Blocks typically occur due to missing or misconfigured SPF, DKIM, or DMARC authentication records, a sudden spike in outbound email volume that triggers anti-spam thresholds, or a compromised account sending malicious content.

How far back can I run a message trace in Exchange Online?

You can run an immediate message trace for emails sent or received within the last 7 days. For messages between 8 and 90 days old, you must submit a request for an extended trace, which will generate a downloadable CSV report, usually within a few hours.

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