5.6.11 Microsoft bounce: invalid characters

What 5.6.11 means

The message contains bare line feed characters, which Exchange Online refuses because they break SMTP framing.

Message text

Invalid characters

Common causes of 5.6.11

• Mail client or script added bare LF line endings instead of CRLF

How to fix 5.6.11

1. Configure the sending software to use CRLF line endings.
2. Check the message source with email header analyzer to see where the bad characters appear.

Check your domain

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Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.

How to read 5.6.11

The second part, 5.6.11, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 5 is a Permanent failure (5xx): the command was rejected and the sender should not repeat it unchanged (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 6 is message content or media: the message content could not be accepted. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.

Do not keep resending the same message: a permanent failure repeats until the cause is fixed. Correct the cause, then send a new message.

Official source: learn.microsoft.com. Last verified 2026-10-20.

Related guides

• How to create an SPF record (syntax and examples)
• How DKIM works and how to publish a DKIM record
• How to set up a DMARC record, from p=none to p=reject

Related codes

• 554 5.6.0 Gmail: malformed message
• 550 5.1.1 Microsoft: bad mailbox address
• 550 5.1.10 Microsoft: recipient not found
• 5.4.1 Microsoft: relay access denied
• 5.4.1 Microsoft: recipient rejected
• 5.4.14 Microsoft: routing loop

FAQ

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