Outlook Error Codes

Fix Exchange NDR Error 550 5.6.0 ConversionFailedException in Outlook

Maira Mehtab Sep 20, 2026 869 views

Question details

The user is experiencing an NDR error 550 5.6.0 ConversionFailedException when transport journaling rules send messages to an external smarthost.

Product: Microsoft Exchange
Device & OS: not provided
Scenario: Routing emails through transport journaling rules to an external smarthost.
Observed behavior: Some messages return an NDR stating M2MCVT.StorageError.Exception: ConversionFailedException due to MIME conversion corruption, while others are delivered successfully.

Before you start

Verify your Office 365 service health dashboard for any ongoing Exchange server incidents, as this specific MIME conversion error can sometimes be caused by temporary backend glitches.

Solution 1Recommended

Investigate MIME Conversion and Message Content

Identify if specific attachments or email formatting are triggering the MIME conversion failure on the Exchange server.

Because the NDR indicates that message content became corrupted during MIME conversion, identifying the exact payload that triggers the failure is critical. This usually involves inspecting the message format and attached files.

Review the NDR details

Open the non-delivery report in your administrator inbox and locate the specific message ID that triggered the M2MCVT.StorageError.Exception.

Compare successful and failed messages

Look for patterns in the failed messages, such as specific attachment types (like oversized PDFs or archives) or unusually complex HTML formatting, and compare them against successfully journaled messages.

Check external smarthost logs

Access your external smarthost administrative console to see if it rejected the MIME format before the Exchange server generated the NDR.

Intermittent Issue: Many administrators report this issue occurring suddenly across multiple tenants and resolving itself without any manual changes to Office 365 or client-side applications.

Solution 2

Monitor the Environment for Recurrence

Since the issue frequently resolves on its own, set up monitoring to track if the error persists or fluctuates over time.

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Frequently Asked Questions

What does NDR 550 5.6.0 ConversionFailedException mean?

This error indicates that the email message content became corrupted during the MIME conversion process on the Exchange server, preventing the system from delivering it to the destination or journaling smarthost.

Why does the 550 5.6.0 error happen intermittently?

The error can be triggered by specific, unsupported message formats or temporary server-side glitches in Office 365, which is why it often appears suddenly and resolves itself without administrator intervention.

Can client-side Outlook settings cause the ConversionFailedException?

Generally, this is a server-side transport or journaling issue. However, emails sent with heavily corrupted attachments or non-standard formatting from a desktop client might trigger the MIME conversion failure on the server.

How do I isolate the cause of the M2MCVT.StorageError.Exception?

Start by running a message trace and comparing failed and successful messages to isolate problematic attachments or formatting. If the issue is widespread across your tenant, check the Office 365 Service Health dashboard, as it may require a backend fix from Microsoft.

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