What 550 5.7.1 means
5.7.1 is RFC 3463's 'Delivery not authorized, message refused'. It's the most overloaded code in email: any per-host or per-recipient filter can return it. The meaning is entirely in the text that follows.
At Gmail, 550 5.7.1 can mean the message looked like spam, the sending IP or domain has very low reputation, the message isn't RFC 5322 compliant (missing or duplicate From, missing Message-ID), or a Workspace admin policy blocked it. At Microsoft it can mean you aren't allowed to send to that recipient or group, or, for Outlook.com consumer addresses, that your IP is on Microsoft's block list (S3150).
What the bounce looks like
The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.
550 5.7.1 This message is likely unsolicited email. To reduce the amount of spam sent to Gmail, this message has been blocked.550 5.7.1 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.550 5.7.1 Messages missing a valid Message-ID: header are not accepted.550 5.7.1 Unfortunately, messages from [IP] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).Outlook.com / Hotmail consumer block on the sending IP.
550 5.7.1 Delivery not authorizedMicrosoft 365: the recipient (often a distribution group) doesn't accept mail from you.
Why cold emailers see 550 5.7.1
- Spam filtering: 'This message is likely unsolicited email', often after complaints or on content that matches spam patterns.
- Very low IP or domain reputation, typically from new domains sending too much, too soon.
- Your IP is on Outlook.com's block list ('part of their network is on our block list (S3150)').
- A malformed message: no From header, multiple From addresses, missing Message-ID or duplicate headers. Usually a homemade sending script.
- A recipient-side rule: a distribution group that only accepts internal mail, or an admin rule that blocks your domain.
How to fix 550 5.7.1
- Read the text. Pick the branch: spam/reputation, blocklist, malformed message, or recipient policy.
- Spam or reputation: pause the campaign, cut volume, drop links and attachments, remove unengaged contacts, and check Google Postmaster Tools.
- S3150 or another blocklist: request delisting (Microsoft's sender support / delist portal for Outlook.com; the blocklist's own form for Spamhaus etc.) after fixing what got you listed.
- Malformed message: fix your sending code so every message has exactly one From address and a valid Message-ID.
- Recipient policy: you can't fix this from outside. Reach the person another way.
Retry or suppress?
It depends on the text. Spam and reputation blocks are about you, not the address, so don't suppress the contact; fix your sending first. Recipient-policy rejections won't change on retry, so stop emailing that address.
Reading the code
The basic code 550 means "Mailbox unavailable or message refused (permanent)" under RFC 5321. In 5.7.1, the 5 is the class (permanent failure) and the 7 is the subject (security or policy). The detail number 1 is registered in RFC 3463 / the IANA registry as "Delivery not authorized, message refused".
- Class 5
- Permanent failure
- Resending the same message won't work. Something about the message, the recipient or the sender has to change first.
- Subject 7
- Security or policy
- A security or policy decision: spam filtering, authentication, blocklists, permissions.
- Detail 5.7.1
- Delivery not authorized, message refused
Check the rest of your setup
Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.
Got a different bounce? Paste it into the SMTP error code lookup to decode it.