550 5.7.1 — Delivery not authorized, message refused
Retrying will not help. The message will not be delivered.
The reply
550 5.7.1 Delivery not authorized, message refused5.7.1 is the most generic refusal in SMTP: the receiving server will not take this message from you, for a policy reason it may or may not name. The words after the code carry the meaning, and they usually point at one of three things: you tried to relay without logging in, your IP or domain is blocked, or the recipient does not accept your mail.
What it means
RFC 3463 defines X.7.1 as "delivery not authorized, message refused" — a security or policy decision. Servers put very different situations under it: an unauthenticated client asking to send to an outside domain (relaying denied), a blocklisted IP, an SPF or DMARC failure, a mailbox that accepts only listed senders. Yandex even uses it for an address that does not exist: "550 5.7.1 No such user!". Read the text after the code before changing anything, and if the receiving provider has its own page in this reference (linked below), use that one.
Why it happens
- Relaying denied: the client did not authenticate, so the server refused to pass mail on to a domain it does not host.
- The sending IP or domain is on a blocklist the receiver uses. The reply often names it: Spamhaus, Barracuda, SpamCop.
- SPF fails, DKIM is missing or DMARC does not align, and the server reports it under the generic code.
- A rule on the recipient's side: the mailbox accepts mail only from approved senders, or only from inside the organisation.
- At Yandex, "No such user!": the mailbox does not exist.
- Content or reputation filtering. Yandex, Gmail and Outlook word these refusals in their own way, and each has its own page.
What to do
- Read the whole reply: "relaying denied", a blocklist name, "SPF" and "No such user" each lead to a different fix.
- For relaying denied, turn on SMTP authentication in the client and send on port 587 or 465 with a login and password.
- Check the sending IP against the public blocklists, and request delisting once the cause is fixed.
- Check SPF, DKIM and DMARC for the From: domain before sending again.
- For "No such user", correct the address or remove it from the list. Retrying will not help.
- If the refusal came from a corporate server, ask the recipient's administrator to allow your domain. Only they can see which rule fired.
Questions
Is 550 5.7.1 a spam block?
Sometimes. The code says only that the server refused on policy. A spam block usually says so in words — spam, unsolicited, reputation. Relaying denied, "No such user" and blocklist hits arrive with the same code for unrelated reasons.
What does "550 5.7.1 No such user" mean?
Yandex, and mail hosted on Yandex 360, answers 5.7.1 instead of the usual 5.1.1 when the address does not exist. Check the spelling with the recipient. It is a recipient-side failure, and sending again will not change it.
Should I retry after 550 5.7.1?
Not unchanged. A 5xx reply is permanent: the same message from the same sender gets the same answer. Fix the cause the text names, then send again.
Why do I get 5.7.1 when sending through my own server?
If the refusal arrives the moment you submit rather than as a bounce later, it was your own server that refused. The client did not authenticate, or logged in with an account that is not allowed to send for this domain.