What it looks like in your logs
550 5.7.1 Service unavailable, Client host [203.0.113.10] blocked using bl.example.org. To request removal from this list see https://bl.example.org/lookup/203.0.113.10What does 5.7.1 mean?
RFC 3463 defines X.7.1 as delivery not authorized, message refused. The receiving server has made an explicit decision not to take your message. Unlike 5.1.1, the address is often perfectly valid. The server simply does not want this message from this sender, or the recipient's settings do not allow it. Your server will not retry it, because the decision is final until something about your sending changes.
The reasons fall into a few groups: your IP or domain is on a blocklist or has poor reputation, your content looked like spam or phishing, authentication failed, or the recipient organization blocks you or only accepts mail from a set list of senders. When 5.7.1 hits many recipients at one provider, look at your own reputation and infrastructure first.
Gmail returns 550 5.7.1 when it considers a message likely unsolicited or when a recipient policy prohibits it. Microsoft 365 returns 550 5.7.1 for blocklisted hosts and for "Unable to relay" when a server will not relay for you. Yahoo uses 5.7.1 and related 5.7.x codes for permanent reputation blocks.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 5.7.1 | ✓ this code |
Common causes
- Your sending IP is on a public blocklist or the provider's internal list.
- Domain or IP reputation at the provider is very low after complaints or spam-trap hits.
- The content or links triggered spam or phishing filters.
- SPF, DKIM or DMARC failed or are missing.
- The recipient or their organization blocks your address, domain or all external senders.
- At Microsoft, you tried to relay through a server that does not accept your mail.
How to fix 5.7.1
- Check your IP and domain with the blacklist checker; if listed, follow the blacklist removal guide.
- Confirm SPF, DKIM and DMARC all pass with the SPF checker, DKIM checker and DMARC checker.
- Review Google Postmaster Tools, Microsoft SNDS and the sender reputation checker for reputation and complaint data.
- Remove unengaged and suspect addresses, and stop mailing any list segment with an unclear opt-in source.
- Audit content for risky links, shorteners and misleading subject lines; see why emails go to spam.
- If one organization blocks you, contact its postmaster or the recipient with the full bounce text.
5.7.1 questions
What does 550 5.7.1 mean?
It means the receiving server refused to deliver your message for a policy or security reason. The address may be fine. The most common causes are a blocklisted IP, poor sender reputation, spam-like content, failed authentication, or a recipient that blocks external mail.
What does 550 5.7.1 mean in Outlook or Microsoft 365?
Microsoft uses it for several situations: your sending IP is on a blocklist, the recipient only accepts mail from approved senders, or you tried to relay through a server that does not permit it. The text after the code says which. For IP blocks, check the blacklist checker and use the Microsoft delisting portal.
Does 5.7.1 mean my IP is blacklisted?
Not always, but it is one of the most frequent reasons. If the text names a list such as Spamhaus, it is. If it mentions policy, relay or a specific sender, it is something else. Our blacklist checker checks the major lists in one go.
What is the difference between 5.7.1 and 4.7.1?
4.7.1 is temporary: the server wants you to retry, often because of greylisting or a missing PTR. 5.7.1 is permanent: retrying will not help until you fix the cause. Never suppress addresses on 4.7.1, and never blindly retry 5.7.1. Track the two rates separately.