Fix email delivery issues for error code 5.6.11 in Office 365

Getting an error when you send an email can be frustrating. If you get a "bounce" message, also called a non-delivery report (NDR), for error code 5.6.11, this topic can help you fix the problem and get your message sent.

I got this bounce message. Help me fix it

I'm an Admin. How can I fix this?

Why did I get this bounce message?

You received this bounce message with error code 5.6.11 because your email program added invalid characters (bare line feed characters) into a message you sent.

I got this bounce message. Help me fix it

If you sent an email and got this bounce message in response, try doing the following:

1. Use a different email program, one that doesn't add bare line feeds to the message, such as Outlook on the web, to send the message.

2. If the original message contained an attachment, try sending the message without the attachment.

If the problem continues, send the bounce message to your email Admin for assistance and refer them to the I'm an Admin. How can I fix this? section below.

I'm an Admin. How can I fix this?

There are a few ways to fix this problem:

1. Make sure the sender tried the steps listed above.

2. The recipient's email admin may be able to do the following:

   1. Change a setting on their email server.

      Some email servers support the ability to enable or disable bare line feed rejection. Exchange Server, for example, includes the Receive connector setting BareLinefeedRejectionEnabled. If the email recipient is using Exchange Server, their admin could disable the BareLinefeedRejectionEnabled setting on the server's receive connector. See the article Set-ReceiveConnector for more information.

   2. Upgrade their legacy email server to a newer version (or an alternative email server) that supports the SMTP protocol BDAT command.

Upgrading the recipient's existing email server to one that supports the SMTP protocol BDAT command can enable messages with bare line feeds to be received. Most modern email servers support BDAT; however, some free and older email servers don't support this feature.

For more information

For more information, read this FAQ about bareline feed email messages.

What's a bare line feed in email messages?

Usually each line of characters in an email message ends with a carriage return followed by a line feed (CR LF). A bare line feed is a line feed (LF) character that is not immediately preceded by a carriage return (CR) character. In other words, instead of a line ending with CR LF, it ends with only LF.

This worked fine before, why did it stop working recently?

Until recently, Office 365 automatically removed bare line feed characters from mail to help it get delivered to recipients using older email servers that don't support chunking and the BDAT command. The support for BDAT is required by the email industry standards referenced in RFC 1830. In an effort to better support security standards, like DomainKeys Identified Mail (DKIM), Office 365 no longer removes bare line feeds from messages. As a result, messages sent to users with older mail servers may be more likely to be rejected.

Still need help?

See Also

Email non-delivery reports in Office 365

RFC 6376 DomainKeys Identified Mail (DKIM) Signatures

RFC 1830 SMTP Service Extensions for Transmission of Large and Binary Mime Messages (BDAT Support)