Microsoft 365 Error Codes

How to Fix Microsoft 365 Error 554 5.6.0 Invalid Message Content

Kushani Nimanthika Oct 7, 2026 868 views

Question details

The user is trying to resolve an issue where outgoing emails are being rejected with an NDR stating error 554 5.6.0, Invalid message content.

How to Fix Microsoft 365 Error 554 5.6.0 Invalid Message Content

Product: Microsoft 365
Device & OS: not provided
Scenario: Sending a daily report or standard email via Microsoft 365 Exchange.
Observed behavior: The email is rejected after several hours by the DLP Policy Agent with error code 554 5.6.0, even though no obvious blocking policy is configured.

Before you start

Before troubleshooting, ensure you have administrative access to the Microsoft Purview compliance portal and a copy of the exact Non-Delivery Report (NDR) received by the sender.

Solution 1Recommended

Review DLP Policies and Test Message Content

Use this method to identify if a hidden or overly broad Data Loss Prevention (DLP) policy is mistakenly flagging your daily report's text or attachments.

Error 554 5.6.0 is almost exclusively generated by the DLP Policy Agent when it detects sensitive information (like financial data, PII, or restricted keywords) in the message body or an attached file. Even if you haven't explicitly set up a new policy, default organizational rules or updated compliance templates might be active.

Review Active DLP Policies

Log in to the Microsoft Purview compliance portal. Navigate to 'Data loss prevention' > 'Policies' and review all active rules that might apply to the sender.

Run a Message Trace

Open the Exchange admin center, go to 'Mail flow' > 'Message trace', and run a trace for the affected sender to pinpoint exactly which DLP rule dropped the email.

Analyze the NDR and Audit Logs

Check the full Non-Delivery Report (NDR) and the compliance audit logs for specific attachment names or sensitive information types that triggered the block.

Test with Redacted Content

Create a test version of your daily report. Remove all attachments and redact any potentially sensitive text, then attempt to send the email again to verify if the content was the sole trigger.

Review DLP Policies and Test Message Content

Policy Propagation Delay: If you recently modified or disabled a DLP policy, keep in mind that changes can take several hours to fully propagate across the Microsoft 365 environment.

Solution 2

Escalate to Microsoft 365 Support

If testing confirms no sensitive content is present and no DLP policies explain the block, escalate the issue for server-side investigation.

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Frequently Asked Questions

What triggers the DLP Policy Agent to return error 554 5.6.0?

This error is typically triggered when an outbound email contains text or attachments that match your organization's Data Loss Prevention (DLP) policies, such as credit card numbers, social security numbers, or confidential document labels.

Why did it take several hours to receive the Non-Delivery Report (NDR)?

Sometimes emails are queued for inspection or experience processing delays within the Exchange Online Protection (EOP) or DLP scanning engines. If the system struggles to scan a complex attachment, it may eventually time out and reject the message, generating a delayed NDR.

How can I bypass a DLP policy to send my daily report?

You should not bypass organizational security policies without authorization. Instead, review the policy requirements, encrypt the document if permitted by your IT department, or remove the flagged sensitive content from the email body and attachments.

Can harmless attachments trigger the Invalid Message Content error?

Yes. If an attachment is corrupted, highly nested (like a zip file inside a zip file), or formatted in a way that the DLP scanner cannot parse, the scanning agent may block the email out of caution, resulting in error 554 5.6.0.

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