Problem Statement

Emails sent from or relayed through Microsoft 365 to SorbSecurity may fail to deliver when the email message contains bare line feeds (LF) instead of the standard Internet email line endings, carriage return + line feed (CRLF).

Microsoft 365 may return the following Non-Delivery Report (NDR):

550 5.6.11 SMTPSEND.BareLineFeedsAreIllegal

This article explains why the error occurs and how Microsoft 365 administrators can troubleshoot and resolve the issue using an Exchange Online mail flow rule.

Solution

Follow the steps below to identify the issue and apply the recommended workaround.

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Step 1 – Verify the NDR Error

When an email cannot be delivered, obtain the Non-Delivery Report from Microsoft 365 and check the error details.

Procedure

1. Open the NDR received by the sender.
2. Locate the diagnostic information or remote server response.
3. Verify whether the error contains:550 5.6.11 SMTPSEND.BareLineFeedsAreIllegal

Expected Result

The NDR identifies the email as having illegal bare line feeds.

If the Error Is Different

Continue with the standard inbound email troubleshooting procedure to investigate DNS, SMTP connectivity, message trace, and email processing.

Refer to How To Troubleshooting Inbound Email Not Received.

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Step 2 – Understand the Root Cause

Internet email messages are expected to use CRLF (\r\n) to terminate lines. A bare line feed uses LF (\n) without the preceding carriage return.

Microsoft 365 applies strict SMTP handling when messages contain bare line feeds. Where CHUNKING is required to preserve the message content, a receiving gateway that does not advertise the 250-CHUNKING SMTP capability may be unable to accept the message through the required transmission method.

SorbSecurity strengthens SMTP smuggling protection to prevent malformed SMTP message boundaries from being exploited. Consequently, Microsoft 365 may be unable to deliver affected messages through the standard SMTP DATA command.

Typical Symptoms

• Microsoft 365 generates an NDR with error 550 5.6.11 SMTPSEND.BareLineFeedsAreIllegal.
• The recipient does not receive the email.
• The message may not appear in SorbSecurity message trace because delivery failed before the gateway accepted it.
• Other emails from the same sender may be delivered successfully if they do not contain the malformed line endings.

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Step 3 – Configure an Exchange Online Mail Flow Rule

A practical workaround is to configure an Exchange Online mail flow rule that appends a small disclaimer to affected messages. This causes Exchange Online to process the message body to apply the disclaimer and may normalize the malformed line endings.

Important: This workaround should be tested with the affected message flow first. Microsoft 365 behavior can vary depending on the message format and processing path. Applying a disclaimer is not a guaranteed fix for every malformed message.

Procedure

1. Sign in to the Exchange Admin Center.
2. Navigate to Mail flow → Rules.
3. Create a new mail flow rule.
4. Enter a descriptive name, such as Normalize Bare Line Feeds for SorbSecurity.
5. Configure the condition to match the affected messages. For example:
   • The sender is a specific sender; or
   • The sender domain is a specific domain.
6. Under the action, select Apply a disclaimer to the message → Append a disclaimer.
7. Enter a small, unobtrusive disclaimer, such as a single period (.).
8. Configure the fallback action if the disclaimer cannot be applied, according to your organization’s mail flow requirements.
9. Save and enable the rule.
10. Send a new test email through the affected route to a recipient protected by SorbSecurity.

Expected Result

If the workaround successfully normalizes the message, Microsoft 365 should be able to deliver it to SorbSecurity without generating the original bare-line-feed error.

If the Issue Persists

• Verify that the mail flow rule matches the affected messages.
• Check the Exchange Online message trace to confirm that the rule was applied.
• Review the latest NDR and diagnostic information.
• Confirm whether the message still contains bare line feeds after Exchange Online processing.
• Escalate the issue to SorbSecurity support with the relevant diagnostic information.

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Step 4 – Verify Email Delivery

After applying the workaround, verify that the email reaches SorbSecurity and is delivered to the intended recipient.

Procedure

1. Send a test email from Microsoft 365 using the affected sender or mail flow.
2. Review the Exchange Online message trace.
3. Search for the email in the SorbSecurity Message Trace → Mail Query.
4. Review the message lifecycle and confirm the final processing status.
5. Verify receipt in the destination mailbox.

Expected Result

• Exchange Online reports successful delivery to the SorbSecurity gateway.
• The email appears in SorbSecurity message trace.
• The email passes the applicable security and policy checks.
• The destination mail server accepts the message.

If the message reaches SorbSecurity but is subsequently blocked or rejected, investigate the message lifecycle and the corresponding SMTP response.

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Information to Collect Before Escalation

If the issue cannot be resolved, collect the following information:

• Sender and recipient email addresses.
• Email subject and Message-ID.
• Date and time of the failed delivery, including the time zone.
• Complete NDR and diagnostic error details.
• Exchange Online message trace results.
• Relevant mail flow rule configuration and execution results.
• SorbSecurity Message Trace results, if available.
• SMTP logs or diagnostic information from the sending and receiving systems, if available.

Provide this information to SorbSecurity support for further investigation.

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Summary

Step	Action	Expected Outcome
1	Verify the NDR error	Confirm 550 5.6.11 SMTPSEND.BareLineFeedsAreIllegal
2	Understand the root cause	Identify malformed email line endings and SMTP capability requirements
3	Configure an Exchange Online mail flow rule	Test whether disclaimer processing resolves the malformed line endings
4	Verify email delivery	Confirm successful delivery through SorbSecurity to the destination mailbox

Note: Do not disable SMTP smuggling protection as a workaround. The recommended approach is to correct or normalize the malformed message at the sending or intermediate mail system while maintaining the receiving gateway’s security controls.