SMTP error 551: What it means and how to fix it
SMTP error 551 overview:
- SMTP 551 is a permanent bounce. Delivery won’t succeed until something changes.
- It often signals forwarding issues. It can also be reputation-related.
- Follow the bounce message. It usually tells you what to fix.
SMTP error 551 indicates the recipient isn’t local to the receiving server, or the message was rejected due to forwarding or reputation issues. This permanent failure requires addressing the specific cause before delivery can succeed.
Sendmarc helps you implement and continuously monitor SPF, DKIM, and DMARC so mailbox providers can reliably verify your domain – strengthening trust signals that can improve sender reputation and reduce blocks over time.
What SMTP error 551 means
A 551 error traditionally means “User not local; please try <forward-path>.” The receiving server is saying it doesn’t handle email for that recipient and is suggesting an alternative delivery route.
In modern email systems, 551 errors often indicate different issues, including domain reputation problems or forwarding configuration errors. The specific meaning depends on the provider.
Common causes of SMTP error 551
- Domain blocklisting: Your sending domain is on a blocklist
- Forwarding misconfiguration: The email forwarding setup is incorrect
- Invalid domain: The recipient’s domain wasn’t found in the database
Provider-specific 551 errors
ImprovMX
| Error message | Meaning |
|---|---|
| 551 5.7.1 – Due to the very low reputation of the sending domain, this message has been blocked | Your domain is on a reputation blocklist |
| 551 5.7.1 – Domain not found | The domain wasn’t found in the database |
| 551 5.7.1 – Unable to correctly read the forwarding recipient | Forwarding configuration is invalid |
Yandex
| Error message | Meaning |
|---|---|
| 551 5.3.0 – Unknown error | General failure in processing |
How to fix SMTP error 551
For domain reputation issues
If the error mentions low reputation or blocklisting:
Step 1: Check blocklists
Use a blocklist checker to confirm whether your sending domain or IP appears on any major blocklists.
Step 2: Request removal
- Identify why you’re listed
- Fix the underlying problem
- Submit a delisting request
- Wait for processing
Step 3: Prevent future listings
- Authenticate all messages with SPF, DKIM, and DMARC
- Maintain clean mailing lists
- Monitor complaint rates
- Only send to opted-in recipients
For forwarding configuration issues
If the error indicates forwarding problems, have the recipient (or their IT team) verify:
- The forwarding address is valid and active
- The forwarding rule points to the correct destination
- There is no forwarding loop
For domain not found errors
If the error indicates the domain wasn’t found:
Step 1: Verify domain configuration
- Check for typos in the address
- Check the DNS for the domain
Step 2: Contact the domain owner
- Reach out to the recipient through other means
- Ask them to verify their email configuration
Email authentication to prevent 551 errors
Proper authentication helps prevent reputation-based 551 rejections.
SPF
SPF tells receiving servers which IPs can send on behalf of your domain.
Example SPF record:
| Host | Type | Value |
|---|---|---|
@ | TXT | v=spf1 ip4:192.168.0.1 include:mail.example.com -all |
DKIM
DKIM cryptographically signs your emails to prove message authenticity.
Example DKIM record:
| Host | Type | Value |
|---|---|---|
selector._domainkey.yourdomain.com | TXT | v=DKIM1; k=rsa; p=[public key] |
DMARC
DMARC provides policy enforcement and reporting.
Example DMARC record:
| Host | Type | Value |
|---|---|---|
_dmarc.yourdomain.com | TXT | v=DMARC1; p=reject; rua=mailto:[email protected]; fo=1; |
Protect your sending reputation with Sendmarc
A 551 error can mean your email was blocked because your sending domain is blocklisted. Sendmarc helps you improve and maintain your sending reputation by strengthening the authentication signals mailbox providers use to trust your domain:
- Implement and align SPF, DKIM, and DMARC across all senders
- Continuously monitor authentication so trust doesn’t degrade over time
- Use DMARC reporting to pinpoint failing sources and fix alignment gaps fast
- Progress to p=reject to reduce spoofing risk and protect brand trust
- Produce credible, board-ready reporting that shows ongoing improvement
Protect your reputation before delivery suffers – get visibility and control across every sending source.
SMTP error 551 FAQs
Is SMTP error 551 common?
SMTP error 551 isn’t one of the most common bounce codes. You will usually see 550 or 554 more often in day-to-day delivery troubleshooting.
Is SMTP error 551 a permanent failure?
Yes. SMTP 551 is a 5xx error, which means it’s treated as a permanent rejection. Your server won’t keep retrying automatically – something needs to change before resending.
What should I check first when I get an SMTP 551 bounce?
Check the full bounce message for clues like “user not local,” “forwarding,” or “reputation.” That wording usually points to where the failure is happening and who needs to fix it.
Can I fix a 551 error caused by the recipient’s forwarding?
You can’t fix a 551 triggered by the recipient’s forwarding from your side. The practical next step is to notify the recipient (using another channel) so they can correct the forwarding issue.