550 5.7.1
Message rejected as spam or policy violation
The receiving server's content or reputation filter rejected the message outright.
550 5.7.1 Message rejected due to content restrictions
What it means in practice
5.7.1 is the catch-all "delivery not authorised" code. Different providers use it for very different reasons, so the text after the code matters more than the code: read the full response line before changing anything.
The common variants: a relay refusal (you tried to send through a server that won't relay for you, usually missing SMTP authentication), a blocklisted sending IP, a recipient that only accepts mail from listed senders, and content or reputation filtering. A relay refusal shows up on every message sent through that server. A blocklist or reputation problem shows up at some receiving providers and not others.
Because a 5xx is permanent, don't retry the same message unchanged. Fix the cause first, then resend.
Common causes
- Sending domain/IP has poor reputation.
- Message content tripped a spam filter (spammy phrasing, broken HTML, no unsubscribe on bulk mail).
- Missing or failing SPF/DKIM/DMARC.
What to do
- →Confirm SPF, DKIM, and DMARC all pass for the sending domain.
- →Check the sending IP/domain isn't listed on a public blocklist (Spamhaus, Barracuda, etc).
- →Review message content for spam-trigger patterns if this is happening broadly, not to one recipient.