SMTP 550 5.7.1: delivery not authorized (message refused by policy)
A 550 5.7.1 reply means the receiving server refused your message on policy grounds. RFC 3463 defines X.7.1 as "Delivery not authorized, message refused". The message was blocked by a rule, not because the mailbox is missing.
The enhanced status code 5.7.1 is defined by RFC 3463 as "Delivery not authorized, message refused": the receiving server accepted the address but a security or policy rule stopped delivery.
Common causes
Relay denied: you tried to send through a server that will not relay for your address.
Authentication policy: SPF, DKIM, or DMARC enforcement rejected the message.
Recipient-side rule: a local block on the sender, domain, or content refused it.
How to fix it
Read the text after the code, since it usually names the exact rule. If relay was denied, authenticate to your own outbound server instead of a third-party host. If an authentication policy blocked you, align SPF and DKIM with your sending domain so DMARC passes. Because the leading digit is 5, this is a permanent failure: the same message will keep being refused until the underlying policy issue is fixed.
Common questions
Does 550 5.7.1 mean the mailbox does not exist?→
No. The address was accepted; a policy or security rule refused delivery. A missing mailbox returns 5.1.1 instead.
Is 550 5.7.1 permanent?→
Yes. The leading 5 marks a permanent failure, so the same message will keep being refused until the policy issue is fixed.
What causes a 5.7.1 relay-denied error?→
Sending through a server that will not relay for your address. Authenticate to your own outbound server, or use one authorized for your domain.
Can SPF or DMARC trigger 550 5.7.1?→
Yes. When a receiver enforces SPF, DKIM, or DMARC, a failing check can be refused with 5.7.1. Align your authentication with the sending domain.