Microsoft mail servers 550 5.6.11
Message format
The provider rejected the message structure. Check headers, MIME formatting, line endings, and required message fields before sending again.
Decision
Do not retry unchanged. Fix the sender setup first.
Confidence: high. Recipient validity: unknown.
Match this reply
- SMTP stage
- End of DATA
- Endpoint
- Inbound MX
bare line feedInvalid charactersStart here
- 1
Validate the message headers and MIME structure before sending again.
- 2
Check for malformed From, Sender, Date, Message-ID, or line-ending issues.
Confirm the cause
- Validate headers, MIME boundaries, line endings, Date, From, Sender, and Message-ID.
- Send a minimal message from the same SMTP client to isolate formatter issues from content issues.
- Fix the message generator before retrying the original message unchanged.
App handling
- Treat this as a sender setup incident, not a bad recipient.
- Hold similar mail for the same sender domain or provider until the fix is verified.
- Retry after a fresh single-recipient test succeeds.
Avoid
- Do not retry unchanged in a loop.
- Do not suppress the recipient unless the text explicitly says the mailbox is invalid.
- Do not treat a permanent SMTP code as proof that the address is bad; policy and routing failures can also be permanent.
Common causes
- Required headers are missing, malformed, duplicated, or contradictory.
- MIME boundaries, line endings, or message structure are invalid.
- The message was generated by a client that does not follow SMTP or RFC 5322 expectations.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestamp