SMTP codes
SMTP 5.7.1 Delivery not authorized
The server refuses by policy: your IP or domain is blacklisted, the content looks like spam, the recipient blocks you, or relaying is denied.
- Class
- 5xx · Permanent failure
- Retried?
- No, the message bounces
- Defined in
- RFC 3463
What to do
- Look for a blacklist name or URL in the text (Spamhaus, Barracuda, SpamCop).
- Check SPF, DKIM, DMARC and PTR.
- If it says relay denied, your server must authenticate.
How it appears in a bounce
550 5.7.1 Delivery not authorized
Paste the full bounce into the decoder — it finds every code and recipient and says what to check.