SMTP Error 553 occurs when an email server rejects a message because the sender or recipient address is invalid, incorrectly formatted, or not authorised for relay.
This guide will help you:
- Understand why the 553 error occurs
- Identify the specific variation returned by the server
- Correct address formatting or authentication issues
- Restore successful email delivery
Most SMTP 553 errors are simple formatting or configuration mistake.
By the end of this guide, you’ll confidently diagnose and resolve SMTP 553 errors without unnecessary support escalation.
What SMTP Error 553 Means
SMTP 553 is a permanent email rejection (hard bounce).
The receiving server refuses the message because it cannot accept the email address or sender identity.
This typically happens when:
- The email address format is invalid
- The recipient does not exist
- The sending server requires authentication
- The domain is not recognised
- Relay permissions are restricted
Because it is a permanent failure, the server will not retry delivery automatically.
Why This Matters
Ignoring SMTP 553 errors may lead to:
- Failed email delivery
- Broken contact forms
- Increased bounce rates
- Reduced sender reputation
- Automated email system failures
Correctly identifying the cause ensures reliable email delivery without impacting your hosting environment.
Step-by-Step Fix for SMTP 553 Error – Invalid Email Address Format
Follow these steps in order.
1: Confirm the Full Error Message
Open the bounce message and confirm it includes variations such as:
553 Invalid user
553 5.7.1 Not an open relay
553 sorry, no mailbox here by that name
553 user unknown
553 address does not exist
553 domain of sender address does not existIf a different SMTP code appears, refer to our SMTP Error Codes guide.
2: Verify the Email Address Format
Confirm the email address includes:
Check for:
- missing @ symbol
- missing domain
- spaces in the address
- unsupported characters
- double periods
Correct the address and resend the email.
3: Confirm the Recipient Address Exists
Contact the recipient to verify:
- the email address spelling
- whether the mailbox is still active
- if the email address has changed
Update the address before retrying the delivery.
4: Enable SMTP Authentication
Some servers return SMTP 553 when authentication is required but not enabled.
Check your email client settings.
Ensure:
- SMTP Authentication: Enabled
- Use same credentials as incoming mail server
Without authentication, many servers will reject outgoing mail.
5: Check the Sender Domain
Some 553 errors occur if the sender domain:
- does not exist
- is incorrectly typed
- lacks DNS records
Verify that the sender’s email domain is valid and active.
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 server returned a 553 permanent rejection
- the exact variation returned by the recipient server
- the timestamp and delivery attempt details
Quick Fixes for Common SMTP 554 Variations
SMTP 553 errors are permanent rejections, meaning the message cannot be delivered until the issue is corrected.
1. 553 5.7.1 Not an Open Relay
Example:
553 5.7.1 Not an open relay
553 authentication requiredWhat It Means:
The mail server requires authentication before sending messages.
How to Fix It:
- Enable SMTP authentication in your email client
- Use your full email address as the username
- Enter the correct mailbox password
2. 553 Invalid User
Example:
553 invalid user
553 user unknownWhat It Means:
The recipient mailbox does not exist.
How to Fix It:
- Confirm the recipient email address spelling
- Ask the recipient to verify the correct address
- Remove invalid addresses from mailing lists
3. 553 Domain of Sender Address Does Not Exist
Example:
553 #5.1.8 Domain of sender address does not existWhat It Means:
The sending email domain cannot be validated.
How to Fix It:
- Verify the sender domain spelling
- Ensure the domain is active
- Confirm DNS records exist
4. 553 Invalid Email Address Format
Example:
553 address syntax errorWhat It Means:
The email address format does not meet server validation rules.
How to Fix It:
- Remove spaces or special characters
- Ensure the address format is correct
- Re-enter the address manually
Advanced (Optional) Fixes (For Technical Users)
Administrators or Technical Users can diagnose authentication or relay issues at the server-level.
1. Check Exim Mail Logs
grep "553" /var/log/exim_mainlogLook for:
- relay denial responses
- invalid sender domains
- authentication failures
2. Verify Sender Domain DNS
dig example.com MX
dig example.com TXTConfirm:
- domain resolves correctly
- SPF records exist
3. Test SMTP Authentication via Telnet
telnet mail.domain.com 25Then:
EHLO domain.com
AUTH LOGINConfirm that authentication is accepted.
4. Check Mail Server Relay Restrictions
In mail server configuration:
Verify:
- SMTP authentication enabled
- sender domain policies
- relay restrictions
Before You Move On
- Confirm the email address format is valid
- Verify the recipient address exists
- Ensure SMTP authentication is enabled
- Confirm the sender domain is active
If This Didn’t Work
Try these steps:
- Send a test email to another address
- Re-enter the email address manually
- Verify DNS records for the sender domain
- Review delivery logs for additional errors
If the problem persists, contact support with:
- full bounce message
- timestamp
- sender address
- recipient address
Common Mistakes to Avoid
- Sending emails without authentication
- Using invalid sender domains
- Sending to outdated mailing list contacts
Empowering Insight
Most SMTP 553 errors are caused by simple formatting mistakes or missing authentication.
Correcting the address or enabling authentication typically resolves the issue immediately.
FAQs
Q. Will SMTP 553 affect my website?
No, this error only affects the specific email message being sent.
Q. Can the server retry the message automatically?
No, SMTP 553 is a permanent rejection and must be fixed before resending.
Q. Does this mean my domain is blacklisted?
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 553 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!