TL;DR
Permanent refusal because the sender was not authorized for that destination: host or recipient filtering may apply. Fix identity, sending path, or policy before any new attempt. Unresolved authorization issues can hurt sender reputation and deliverability. Do not retry the unchanged attempt.
What this code means
Code 5.7.1 is the class-5 permanent form of an authorization refusal for the named destination. Per-host or per-recipient filtering can yield the same reply, but the code alone does not name the applied rule or system and does not prove that the recipient address is invalid.
Provider examples
550 5.7.1 This message violates example.com email policy. - gcdp <sessionid> - gsmtp550 5.7.1 RESOLVER.RST.AuthRequired; authentication required [Stage: CreateMessage]530 5.7.1 Client was not authenticated550 5.7.1 Recipient doesn't want your mail550 5.7.1 Unfortunately, messages from [31.57.142.204] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [WP1FDK33EO054.eop-nam02.prod.protection.outlook.com 2023-01-01T00:00:00.001Z 03SDKS32D1WER234]550 5.7.1 Unfortunately, messages from [x.xx.xx.xx] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [#.eop-nam02.prod.protection.outlook.com]550 5.7.1 Service unavailable, Helo domain is listed in Spamhaus. To request removal from this list see https://www.spamhaus.org/query/lookup/ (S8001) [#.prod.protection.outlook.com]550 5.7.1 Service unavailable, MailFrom domain is listed in Spamhaus. To request removal from this list see https://www.spamhaus.org/query/lookup/ (S8002) [#.eop-nam02.prod.protection.outlook.com]554 5.7.1 [CS01] Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137550 5.7.1 [CS01] Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137host xx.yahoodns.net [x.xx.xx.xx] SMTP error from remote mail server after MAIL FROM:<user@example.com> SIZE=2022: 553 5.7.1 [BL21] Connections will not be accepted from x.xx.xx.xx, because the ip is in Spamhaus's list; see http://postmaster.yahoo.com/550-bl23.htmlhost xx.yahoodns.net [x.xx.xx.xx] SMTP error from remote mail server after MAIL FROM:<user@example.com> SIZE=2022: 553 Mail from x.xx.xx.xx not allowed - 5.7.1 [BL23] Connections not accepted from IP addresses on Spamhaus XBL; see http://postmaster.yahoo.com/errors/550-bl23.html [550]550 5.7.1 [x.xx.xx.xx] Our system has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to Gmail, this message has been blocked. Please visit https://support.google.com/mail/?p=UnsolicitedMessageError for more information. - gsmtp550 5.7.1 The user or domain that you are sending to (or from) has a policy that prohibits the email that you sent. Contact your domain administrator for assistance. For more information, go to Sorry, a policy is in place that prevents your message from being sent. - gsmtp550 5.7.1 Invalid credentials for relay ip-address. The IP address you've registered in your Workspace SMTP Relay service doesn't match the domain of the account this email is being sent from. If you are trying to relay email from a domain that isn't registered under your Workspace account or has empty envelope-from:, you must configure your email server either to use SMTP AUTH to identify the sending domain or to present one of your domain names in the HELO or EHLO command. For more information, go to SMTP relay service error messages. - gsmtp550 5.7.1 This message is likely unsolicited email. To reduce the amount of spam sent to Gmail, this message has been blocked. For more information, go to Why has Gmail blocked my messages? - gsmtp550 5.7.1 This message does not meet IPv6 sending guidelines regarding PTR records and authentication. For more information, go to Email sender guidelines. - gsmtp550 5.7.1 This message is likely suspicious due to the very low reputation of the sending IP address. To best protect our users from spam, the message has been blocked. For more information, go to Why has Gmail blocked my messages? - gsmtpA 553 or 554 SMTP error indicates an email could not be delivered due to a permanent problem. … Your IP is listed by Spamhaus. Please check with https://www.spamhaus.org.550 SC-001 — Mail rejected by Outlook.com for policy reasons. Reasons for rejection may be related to content with spam-like characteristics or IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help.550 SC-002 — Mail rejected by Outlook.com for policy reasons. The mail server IP connecting to Outlook.com has exhibited namespace mining behavior. If you are not an email/network admin please contact your Email/Internet Service Provider for help.550 SC-004 — Mail rejected by Outlook.com for policy reasons. A block has been placed against your IP address because we have received complaints concerning mail coming from that IP address. We recommend enrolling in our Junk Email Reporting Program (JMRP), a free program intended to help senders remove unwanted recipients from their email list. If you are not an email/network admin please contact your Email/Internet Service Provider for help.550 DY-001 — Mail rejected by Outlook.com for policy reasons. We generally do not accept email from dynamic IP's as they are not typically used to deliver unauthenticated SMTP email to an Internet mail server. If you are not an email/network admin please contact your Email/Internet Service Provider for help. For more information, Spamhaus maintains lists of dynamic and residential IP addresses.550 OU-001 — Mail rejected by Outlook.com for policy reasons. If you are not an email/network admin please contact your Email/Internet Service Provider for help. For more information about this block and to request removal please go to: Spamhaus .550 OU-002 — Mail rejected by Outlook.com for policy reasons. Reasons for rejection may be related to content with spam-like characteristics or IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help.550 5.7.1 [2500:1109:110::11 91] Gmail has detected that this message is likely suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. t64-20020a1fc343000000b0049d0aa186d9si449748vkf.30 - gsmtp<john@zovit.com>: host aspmx.l.google.com[124.205.72.72] said: 550-5.7.1 [93.177.84.58 12] Gmail has detected that this message is likely 550-5.7.1 unsolicited mail. To reduce the amount of spam sent to Gmail, this 550-5.7.1 message has been blocked. Please visit 550-5.7.1 https://support.google.com/mail/?p=UnsolicitedMessageError for more 550 5.7.1 information. ga24-20020a1709070c1800b009e0f32d8e62si964060ejc.893 - gsmtp (in reply to end of DATA command)550 5.7.1 Your message was rejected due to example.com DMARC policy.550 5.7.1 Your email was rejected due to having a domain present in the Spamhaus DBL -- see https://www.spamhaus.org/dbl/554 5.7.1 <example@icloud.com>: Relay access denied554 5.7.1 [CS01] Message rejected due to local policy.554 5.7.1 [HM08] Message rejected due to local policy.554 5.7.1 [BS01] Message rejected due to local policy.smtp;550 5.7.1 Your message was rejected due to example.com’s DMARC policy. See https://support.apple.com/en-us/HT204137 for info550 5.7.1 Service unavailable, Client host [x.xx.xx.xx] blocked using Spamhaus. (AS16012611)550 5.7.1 Service unavailable, Helo domain is listed in Spamhaus. (S8001) [#.prod.protection.outlook.com]550 5.7.1 TRANSPORT.RULES.RejectMessage; the message was rejected by organization policysmtp;550 5.7.1 <email@example.com>: Recipient address rejected: User email address is marked as invalid.smtp;550 5.7.1 Relaying deniedsmtp;550 5.7.1 Service unavailable; client [x.xx.xx.xx] blocked using prs.proofpoint.comsmtp;550 5.7.1 Service unavailable; client [x.xx.xx.xx] blocked using Cloudmark Sender Intelligence (Visit http://csi.cloudmark.com/reset-request/ if you feel this is in error)smtp;554 5.7.1 <email@example>: Relay access deniedTechnical meaning
Unauthorized sending to the destination, with the message refused, is the meaning of X.7.1; per-host or per-recipient filtering may produce this result. The registry description says this detail is useful only as a permanent error, and concrete code 5.7.1 applies it in class 5.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. The code alone does not identify the specific rule or system that applied it, and it does not establish that the recipient address is invalid.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Operational guidance: stop automatic and manual retries of the same unchanged attempt. Consider a new send only after a verified change to the sender identity or authorization, the applicable rule, or the sending path; check suppression again before the attempt.
Suppression decision
Operational guidance: do not automatically add an address or domain to a suppression list based on 5.7.1 alone. Inspect the complete response, authorization and filtering context, event history, and applicable policy, then make the suppression decision in that context.
Common causes
- The reporting system determined that the sender was not authorized to send to the destination, for example because of per-host or per-recipient filtering.
- In the accepted Gmail example, the message violated a custom-domain email policy.
- In the accepted Exchange Online example, delivery from an external sender to a mail-enabled public folder required authentication.
Diagnostic steps
- Inspect the raw SMTP response or nondelivery report and confirm that the enhanced code is exactly 5.7.1 and that the basic reply is in the 5xx class; retain the complete response text.
- Correlate the event with the intended message, attempt time, sender identity and host, destination, transaction stage, and system that returned the code; identify the specific rule from available logs instead of inferring it from the code alone.
- If the response contains Gmail marker “gcdp”, inspect the applicable custom-domain policy; if it contains “RESOLVER.RST.AuthRequired”, inspect the authentication requirement for the specified Exchange Online public-folder delivery.
- Stop unchanged retries; before a controlled new send, verify a material correction and reassess suppression.
Actions by owner
Sender
- Do not resend the same unchanged message; confirm the intended sender identity and destination, then give the administrator the complete response.
- Correct only a confirmed error in the sender, recipient, or required sending path, without treating the address as invalid from this code alone.
Sender administrator
- Retain the complete response and attempt context, then inspect the sender identity, host, authentication, and sending-path configuration used for the specific refusal.
- Stop unchanged retries; after a verified correction, make a controlled new attempt and reassess suppression.
Recipient administrator
- If the code came from a recipient system you manage, inspect authorization rules and host- or recipient-level filters for the specified attempt, including the applicable domain policy or authentication requirement.
- If the confirmed rule does not match the intended policy, correct it within your authority and verify the result of a new, controlled attempt.
Provider
- For the specified attempt, inspect managed-service logs and the applied rule, then give the administrator exact, safe context for the refusal.
- Correct a confirmed problem in the managed layer or identify the owner of the required correction; do not trigger automatic suppression from 5.7.1 alone.
Sources
These sources define what this enhanced status code means, mainly through the IANA registry and related RFCs. When provider examples appear on the page, they come from that provider's published documentation. Follow the links to read the original wording in context.
- SMTP Enhanced Status Codes — IANA registry of enhanced mail system status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- RFC 2034 — SMTP Service Extension for Returning Enhanced Error Codes — Defines how SMTP returns enhanced status codes to clients.
- RFC 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
- Gmail SMTP errors and codes (Google Workspace Help) — Google Workspace Help article on Gmail SMTP error messages.
- 5.7.1 AuthRequired NDR in Microsoft 365 — Microsoft Learn article on the AuthRequired nondelivery report.
- Email nondelivery reports and SMTP errors in Exchange Online — Microsoft Learn overview of Exchange Online NDR and SMTP errors.
- Why the message was not delivered — Interia Mail help article explaining delivery-failure responses.
- email-bounce-parser (test fixture corpus) — Open-source bounce-parsing library whose test fixtures are pinned locally as evidence.
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
- SMTP Field Manual (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
- Gmail SMTP errors and codes — Official Gmail Help table of SMTP error messages and status codes.
- Yahoo SMTP Error Codes — Yahoo Sender Hub guide to SMTP deferral and rejection codes.
- Outlook.com Postmaster troubleshooting — Official Outlook.com Postmaster page with consumer SMTP error codes.
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