SMTP enhanced status code 5.7.1

What the registry says

The basic codes it travels with

• 451 Requested action aborted: local error in processing
• 454 Temporary authentication failure
• 502 Command not implemented
• 503 Bad sequence of commands
• 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
• 551 User not local; please try <forward-path>

In practice

550 5.7.1 is a wide bucket. Gmail uses it for likely spam, very low IP or domain reputation, missing or duplicate headers, and recipient policies. Microsoft uses it for delivery not authorized, unable to relay and client not authenticated. Blocklist rejections that cite Spamhaus commonly arrive as 5.7.1 too. Sort the bounces by text before acting, since each needs a different fix.

What the sender should do

Read the text after the code to see which kind of policy fired. Check SPF, DKIM and DMARC, then blocklists, then whether the recipient only accepts internal mail.

How providers use X.7.1

Quoted from each provider’s own error reference.

• Gmail 550 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.

• Microsoft 365 5.7.1

  Delivery not authorized

Common questions

What does 5.7.1 mean?

X.7.1 means the sender is not authorized to deliver to this recipient, because of per-host or per-recipient filtering. As 550 5.7.1 it is one of the most common policy rejections, covering blocklists, failed authentication, relay denial and recipient restrictions.

What is the difference between 5.7.1 and 4.7.1?

They share the subject and detail digits, so they report the same condition, "Delivery not authorized, message refused". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.

What should I do about 5.7.1?

Read the text after the code to see which kind of policy fired. Check SPF, DKIM and DMARC, then blocklists, then whether the recipient only accepts internal mail.

Would an email verifier have caught 5.7.1?

Partly. The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.

Full guides for this code

• 550 5.7.1 — message refused by policy
• 554 5.7.1 — transaction failed, message rejected as spam
• 571 5.7.1 — delivery not authorized, message refused
• 550 5.7.1 S3140 — IP range on Outlook.com block list
• 451 4.7.1 — Temporarily deferred (greylisting or policy)