What it looks like in your logs
554 5.7.1 Service unavailable; Client host [203.0.113.10] blocked using zen.example.org; see https://www.example.org/lookupWhat does 554 mean?
RFC 5321 defines 554 as transaction failed, and also allows it in place of the greeting to mean "no SMTP service here". It is the most generic permanent failure there is. The receiving server is refusing the whole transaction and has chosen not to be more specific in the basic code. Your server will not retry it, and it should not, because resending the same message usually produces the same result.
Because it is generic, providers use 554 for their most serious rejections: a sending IP on a blocklist, a message classified as spam or malware, or authentication that failed badly. A 550 is often about one address, while a 554 is often about you. When several recipients at the same provider return 554, assume a reputation or infrastructure problem until you have ruled it out.
Microsoft 365 returns 554 5.7.1 for spam and blocked senders and a range of 5.7.x codes for banned IPs. Yahoo uses 554 with tags, including "554 delivery error: dd This user doesn't have an account", which is actually a user-unknown bounce, and 554 5.7.9 for policy rejections. Read the text before deciding what kind of 554 you have.
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 |
| 554 | ✓ this code |
Common causes
- Your sending IP is listed on a public blocklist or the provider's own list.
- The message was classified as spam, phishing or malware.
- SPF, DKIM and DMARC are missing or failing on the message.
- Your domain or IP reputation at that provider is very poor.
- The server rejected the connection at the greeting, before any command.
- At Yahoo, the recipient account does not exist (the "dd" variant).
How to fix 554
- Check the IP and sending domain with the blacklist checker and follow the blacklist removal guide for any listing.
- Verify SPF, DKIM and DMARC with the SPF checker, DKIM checker and DMARC checker.
- Review reputation in Google Postmaster Tools and Microsoft SNDS, and check the sender reputation checker.
- Remove invalid and long-inactive addresses before sending again.
- Audit content for risky links, URL shorteners and misleading subject lines; our guide on why emails go to spam lists the usual triggers.
- Pause sending to the affected provider until the cause is fixed, then resume at lower volume.
554 questions
What does SMTP error 554 mean?
It means the receiving server permanently refused the transaction. The basic code is deliberately vague, so the enhanced code and text carry the real reason. Common reasons are a blocklisted IP, spam or malware detection, failed authentication and poor sender reputation.
What does "554 5.7.1 message blocked" mean?
It means the message was refused on policy or security grounds. Usually the sending IP is blocklisted, the content looked like spam, or authentication failed. The address may be valid. See 5.7.1 for a full breakdown of the causes and fixes.
Can I resend an email after a 554 error?
Not until you fix the reason it was refused. Resending the same message from the same IP will get the same answer and can deepen the block. Clear any listing, fix authentication and content, then send again at lower volume.
What is the difference between 554 and 550?
Both are permanent. A 550 is frequently tied to a single mailbox that does not exist. A 554 more often signals that the receiver distrusts your IP, domain or content, so it tends to affect many recipients at once. Check the enhanced code in both cases.