If your email fails with SMTP Error 551, the receiving server is telling you that the recipient address is invalid, moved, or not hosted on that server.
This guide will help you:
- Understand why the error occurs
- Identify the exact cause of the rejection
- Fix the issue quickly
- Restore successful email delivery
In most cases, this error simply means the recipient email address needs to be corrected or updated.
By the end of this guide, you’ll confidently diagnose and resolve SMTP 551 errors without unnecessary support escalation.
What SMTP Error 551 Means
SMTP Error 551 is a permanent delivery failure (hard bounce).
It usually occurs when:
- The recipient email address no longer exists
- The mailbox has moved to another server
- The address belongs to another domain
- The mail server refuses to relay the message
Because it is a hard bounce, the sending server will not retry delivery automatically.
Why This Matters
Ignoring SMTP Error 551 may lead to:
- Repeated permanent delivery failures (hard bounces)
- Important emails never reaching the intended recipient
- Outdated contacts remaining in mailing lists or CRM systems
- Reduced sender reputation if invalid addresses continue receiving mail
- Unnecessary support tickets when the issue is simply an incorrect or relocated address
Because SMTP 551 is a permanent rejection, the mail server will not retry delivery automatically.
Correctly identifying the reason for the 551 response ensures you update the recipient address or routing information, allowing future emails to be delivered successfully.
Step-by-Step Fix for SMTP 551 Error – Invalid or Relocated Recipient
Follow these steps in order.
1: Confirm the Full Error Message
Open the bounce notification and confirm it includes variations such as:
551 User not local
551 not our customer
551 5.1.1 user does not exist
551 recipient address failed to route
551 relay request denied
551 User not local; please try <forward-path>The extended message often explains why the receiving server cannot deliver the email.
SMTP 551 typically indicates the recipient mailbox does not exist on that server or has been moved elsewhere.
If a different SMTP code appears, refer to the SMTP Error Codes guide.
2: Check the Recipient Email Address
The most common cause is typing mistakes.
Verify:
- Spelling of the mailbox name
- Domain name
- Extra spaces or characters
Example:
- Incorrect: [email protected]
- Correct: [email protected]
3: Confirm the Address Still Exists
Sometimes recipients:
- Delete old mailboxes
- Change email providers
- Move to new domains
Contact the recipient through another method and confirm the correct address.
4: Update Saved Contacts or Mailing Lists
If the address exists in:
- Address books
- Mailing lists
- CRM systems
Update the contact record to prevent future delivery failures.
5: Remove Invalid Recipients from Bulk Emails
If you’re sending emails to many recipients, remove addresses that generate SMTP 551 errors.
Continuing to send to invalid addresses can negatively affect your email reputation.
6: Review Delivery Logs (Basic Level)
Log in to:
- cPanel → Email → Track Delivery
- Plesk → Websites & Domains → Select your domain → Mail → Track Email Delivery
Confirm:
- The message was permanently rejected with a 551 response
- The recipient server returned a routing or user-location error
- The exact timestamp and server response message
Delivery logs help verify whether the problem is caused by:
- A non-existent mailbox
- A recipient that moved to another server or domain
- Relay restrictions or routing configuration on the recipient server
7: Resend the Email
Once the correct address is confirmed:
- Resend the email.
Delivery should be completed successfully.
Quick Fixes for Common SMTP 554 Variations
SMTP 551 is a permanent delivery failure (“hard bounce”). This means the receiving server refused the message because the recipient mailbox is invalid, has moved, or is not hosted on that server.
Unlike temporary errors (4xx), SMTP 551 requires action before the email can be resent successfully.
1. 551 User Not Local
Example:
551 User not local
551 user not local; please try <forward-path>
551 recipient is not localWhat It Means:
The receiving mail server does not host the mailbox you are trying to send to. It simply means the destination mailbox is not located on that server.
When This Typically Happens:
- The mailbox moved to another server
- The email address belongs to another domain
- The address was changed or migrated
How to Fix It:
- Confirm the correct email address with the recipient
- Ask if their email provider or domain recently changed
- Update the address in your contacts or mailing lists
- Resend the message using the correct address
2. 551 Not Our Customer
Example:
551 not our customer
551 mailbox not hosted here
551 domain does not handle this recipientWhat It Means:
The receiving server recognises the domain but does not manage mail for the specific recipient. This error only affects delivery to that specific address and does not impact your hosting services.
When This Occurs:
- The mailbox was deleted
- The domain changed email providers
- The recipient address is outdated
How to Fix It:
- Verify the email address spelling
- Confirm the recipient still uses that mailbox
- Ask for an updated email address
- Update saved contact records
3. 551 Relay Access Denied
Example:
551 Relay access denied
551 This is not a relay host
551 mail must be to or from local domainWhat It Means:
The receiving mail server does not allow messages to be relayed through it.
When This May Happen:
- The message is routed incorrectly
- The server refuses to forward mail to external domains
- SMTP relay policies block the request
How to Fix It:
- Confirm the recipient email domain is correct
- Ensure your SMTP server settings are configured correctly
- Verify SMTP authentication is enabled if required
This does not mean your account is blocked. It simply indicates that the receiving server will not relay the message.
4. 551 Recipient Does Not Exist
Example:
551 5.1.1 recipient does not exist
551 unknown user
551 user not foundWhat It Means:
The email address does not exist on the destination mail server.
When This Usually Occurs:
- The mailbox was deleted
- The address was entered incorrectly
- The recipient organisation removed the account
How to Fix It:
- Double-check the spelling of the email address
- Confirm the mailbox is still active
- Request an alternative contact address if needed
This error only affects that specific email address and does not impact your email service or domain.
Advanced (Optional) Fixes (For Technical Users)
Administrators, or Technical Users can diagnose routing problems using the following checks.
1. Verify Recipient Mail Routing
Use DNS lookup tools to confirm the recipient domain’s MX records.
Example:
nslookup -type=mx example.comThis shows which servers handle mail for that domain.
2. Check SMTP Logs
Mail server logs may show detailed routing errors.
Example:
551 user not local
551 relay access denied3. Confirm SMTP Relay Configuration
If the error is Relay Access Denied, verify:
- SMTP authentication enabled
- Correct SMTP server
- Proper port configuration
Common SMTP Ports:
- 587
- 465
- 25
Before You Move On
Confirm:
- Confirm the recipient address is correct
- Verify the mailbox still exists
- Update saved contacts or mailing lists
- Remove invalid recipients from bulk emails
Then resend the message.
If This Didn’t Work
Try the following:
- Contact the recipient to confirm their new email address
- Check whether the domain has changed
- Verify your SMTP settings if relay errors appear
- Review the full bounce message for additional clues
If the problem continues, contact support and provide:
- Full bounce message
- Sender email address
- Recipient email address
- Date and time the message was sent
Common Mistakes to Avoid
- Sending repeatedly to an invalid address
- Ignoring bounce messages
- Using outdated mailing lists
- Assuming the issue is server-side
Empowering Insight
Email errors like SMTP 551 usually indicate a simple contact problem, not a server failure.
Updating the correct recipient address typically restores delivery immediately.
FAQs
Q. Why did the address stop working?
Possible reasons include:
- The mailbox was deleted
- The recipient changed email providers
- The domain moved to another mail server
Q. Will the email resend automatically?
No, as this is a hard bounce, the sending server will not retry automatically.
Q. Can I undo this error?
Yes, once the correct recipient address is used, email delivery should work normally.
Additional Resources
SMTP Error Codes
SPF/DKIM Setup Guide
Email Sending Limits Guide
Email Bounce Back Guide
Need Additional Support?
We’re Here to Help:
Understanding SMTP Error 551 and how to fix this doesn’t have to feel technical with this easy-to-reference guide. Stuck? Check out our Scope of Support, and then contact our Support Team for further clarity and guidance (https://1grid.co.za/contact-us/). We’re ready to see how we can help!