550 5.7.1: malformed header
Gmail rejected the message because one of its header lines is malformed under RFC 5322; the bounce names which header it is.
- Gmail
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.7.1
- Provider
- Gmail and Google Workspace
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
550 5.7.1 This message is not RFC 5322 compliant. There is a malformed header…
What it means
One header line does not follow the format RFC 5322 sets out: a field name, a colon and a value, with long values folded onto lines that begin with whitespace. Gmail names the header in the bounce. Typical faults are a missing colon, a continuation line without leading whitespace, invalid characters in the field name, and a structured header such as Date, or an address field, whose value cannot be parsed. Messages built by mail libraries rarely have this problem.
Common causes
- Hand-built headers missing the colon, or with spaces in the field name.
- Long header values broken across lines without leading whitespace.
- A Date or address header whose value does not follow the standard syntax.
How to fix it
- 01Read the header named in the bounce in the raw message.
- 02Fix the code or template that writes it, or let a mail library build the headers.
- 03Check folding: every continuation line must start with a space or a tab.
- 04Resend once the header is valid.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com
- Email sender guidelines (Gmail Help)support.google.com
Related codes
- 554 5.6.0Malformed message.Gmail refused the message because its structure breaks the Internet Message Format rules of RFC 5322, so it cannot be read reliably.
- 550Mailbox unavailable.The receiving server permanently refused the message for this recipient: the mailbox does not exist, cannot be reached, or the server rejected the message by policy.
- 5.7.1Delivery not authorised.The receiving server refuses the message because this sender is not allowed to send to this recipient, as a result of filtering by host or by recipient.
5.7.1 at other providers
- 550 5.7.1Not authorized to send to this recipient.You do not have permission to send to this recipient, typically a group restricted to certain senders or a recipient protected by a mail flow rule.
- 550 5.7.1Public folder does not accept external mail.An external sender wrote to a mail-enabled public folder in Microsoft 365, which by default accepts mail only from authenticated senders.
- 550 5.7.1Sending IP blocked by Proofpoint Dynamic Reputation.Proofpoint’s IP reputation system has your sending IP on its block list because it has seen spam or viruses from it, so Proofpoint customers refuse your mail.
- 550 5.7.1SPF check failed.The recipient’s Cisco Secure Email gateway checked SPF, found your server not authorised for the sender domain, and its policy rejects that result.
- 550 5.7.1Rejected under your domain’s DMARC policy.Your message failed DMARC, your domain’s DMARC record asks receivers to reject such mail, and the recipient’s Cisco gateway did so.
- 550 5.7.1DMARC verification failed permanently.The Cisco gateway hit a permanent failure while verifying DMARC for your message, and the recipient’s policy rejects such mail. Cisco does not say what produces that failure.
Further reading
All 108 Gmail codesEvery code in the wikiPaste your own bounce