The receiving server refused the message because of a policy: your IP or domain reputation, a blacklist listing, failed authentication, or content filtering. The text after the code tells you which.
Policy rejection · fixable
Common causes
- Your sending IP or domain is on a blocklist (Spamhaus, Microsoft, Barracuda…)
- SPF, DKIM or DMARC failed
- The recipient blocks your domain or only accepts mail from certain senders
- Content looked like spam or phishing
- Relaying denied — you tried to send through a server that is not yours without logging in
How to fix it
- Paste the full bounce into the Bounce Explainer to see the exact reason
- Check your IP and domain on the Blacklist Checker and request removal after fixing the cause
- Make sure SPF, DKIM and DMARC pass for your sending domain
- If it says "relay denied", configure your mail client to authenticate (SMTP AUTH) on port 587
Blacklist CheckerRun the right check for your domain or paste the full bounce message for a precise explanation.
Open Blacklist Checker →Frequently asked questions
Is 550 5.7.1 a hard bounce?
It is a permanent rejection of that message, but the cause is usually fixable on your side (reputation or authentication). Fix the cause before resending.
Why does Outlook / Hotmail send 5.7.1?
Microsoft uses it when your IP is on its internal block list. The message often includes S3150 and a link to the delisting portal.