550 5.6.11: message contains bare line feeds

What it means

Lines in an email should end with a carriage return followed by a line feed (CR LF). A bare line feed is an LF without the CR. A message that contains them can only be sent with SMTP chunking, the BDAT command from RFC 3030. Microsoft 365 used to remove bare line feeds so older servers could accept such messages, but stopped, to better support standards such as DKIM. So when the destination server lacks BDAT, the message bounces with 5.6.11. Usually the email program that wrote the message added the bare line feeds.

Common causes

• The email program or application that wrote the message used LF instead of CR LF line endings.
• An attachment or generated content introduced bare line feeds.
• The destination server does not support BDAT (SMTP chunking).

How to fix it

1. 01Send the message from a different program, such as Outlook on the web.
2. 02If the message had an attachment, try sending it without the attachment.
3. 03Recipient admins: allow bare line feeds on the receiving server if it has such a setting (on Exchange Server receive connectors, -BareLinefeedRejectionEnabled $false).
4. 04Or move the receiving server to software that supports BDAT.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• Fix NDR error "550 5.6.11" in Exchange Onlinelearn.microsoft.com
• Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com

Further reading

• The standard meaning of 550: mailbox unavailable