Users of Exchange Online or Exchange Online Protection may not receive an expected email message. If you perform a trace on the message, you find that message delivery failed and that a nondelivery report (NDR) that contains the following error message was generated:
SMTPSEND.BareLinefeedsAreIllegal; message contains bare linefeeds, which cannot be sent via DATA
Users may not have experienced this problem in the past when they used Forefront Online Protection for Exchange (FOPE).
This issue occurs if the source message transfer agent (MTA) didn't append the expected CR-LF combination to the end of the message as documented in Request For Comments (RFC) 2822.
To resolve this issue, do one of the following, as appropriate for your situation:
Request that the sender correct the message format. This error frequently occurs with automated mailers (for example, reports or invoices) and bulk senders.
Enable Extended SMTP (ESMTP) on the receiving server so that the message can be sent by using CHUNKING or the BDAT command. Neither of these methods depends on the CR-LF combination to signal the end of the message. For more information, see SMTP protocol extensions.
If the sender cannot correct the sent messages, or if the gap must be bridged until the message format is corrected, the recipient can create an inbound transport rule to append a disclaimer to the messages from the problematic sender. The disclaimer will append the expected CR-LF combination to the message so that it can be delivered. (This disclaimer may consist of a single character such as a period or a dash.)