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SMTP error code · Microsoft 365 / Outlook.com

550 5.6.11: invalid characters (bare line feeds)

550 5.6.11 Permanent failure (5xx): the message bounced
By iBlockSpam Team Checked against sources 4 sources How we check our content
What it means

The message contained bare line feed characters, and the destination server doesn't support the SMTP feature needed to accept them, so Microsoft 365 couldn't deliver it.

What the bounce says
Invalid characters

The description Microsoft's Exchange Online NDR reference gives for 5.6.11.

What 550 5.6.11 means

Microsoft says you get 5.6.11 because your message contains bare line feeds and the destination email server doesn't support messages with bare line feeds. A bare line feed is a line feed character (LF) that isn't immediately preceded by a carriage return (CR). RFC 5321 is strict here: a line in SMTP ends with CR followed by LF, and conforming implementations must not recognize or generate any other line terminator.

According to Microsoft, a message with bare line feeds can only be sent between servers using SMTP chunking, the BDAT command defined in RFC 3030. If the destination doesn't support BDAT, it can't accept the message. Microsoft 365 used to remove bare line feeds so delivery to older servers would work, but to better support security standards such as DKIM, it no longer does.

So this error usually involves two things at once: an email program or script that writes bare line feeds into the message, and an older receiving server without BDAT support. The sender can avoid the first; only the recipient's admin can fix the second.

Likely causes

  • The email program, script or device that created the message used LF instead of CRLF at line ends.
  • An attachment or generated content introduced bare line feeds.
  • The destination server doesn't support the SMTP BDAT command (chunking), so it can't take such messages.

How to fix it

If you sent the message

  1. Send the message from a different email program, such as Outlook on the web, as Microsoft suggests.
  2. If the original had an attachment, try sending without it to see whether the attachment is the cause.
  3. If an app or script generates the mail, fix it to end every line with CRLF.
  4. If that doesn't help, refer the recipient's admin to Microsoft's article on 5.6.11.

If you're the recipient or their admin

  1. Admins can disable bare line feed rejection where the server supports it; on Exchange Server, Microsoft cites -BareLinefeedRejectionEnabled $false on the internet Receive connector.
  2. Or upgrade to mail server software that supports the SMTP BDAT command.
Email Header Analyzer Email Health Check

Frequently asked questions

What is a bare line feed in an email?

A line feed character (LF, ASCII 10) that isn't preceded by a carriage return (CR, ASCII 13). Email lines are supposed to end with CRLF. Some scripts and older programs write only LF, which strict or older servers reject.

Why does 5.6.11 happen now when the same mail used to work?

Microsoft says Microsoft 365 used to strip bare line feeds so older servers could accept the mail, but it stopped doing so to better support security standards such as DKIM. Servers without BDAT support now get the message as written and can refuse it.

Sources

  1. Fix NDR error 550 5.6.11 in Exchange Online (Microsoft Learn)
  2. Email nondelivery reports (NDRs) and SMTP errors in Exchange Online (Microsoft Learn)
  3. RFC 5321, section 2.3.8: Lines
  4. RFC 3030: SMTP Service Extensions for Transmission of Large and Binary MIME Messages (BDAT)

Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.

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