SMTP Error 550: Message Rejected by Server (How to Fix It)

Open the bounce message or email error details. 

Ensure it specifically references variations of: 

550 Requested action not taken 
550 No such user here 
550 Mailbox unavailable 
550 Relay access denied 
550 5.1.1 User unknown 

If the error shows 552, 554, 535, or 421, refer to our SMTP Error Codes guide for more information. 

SMTP 550 often means the mailbox does not exist. 

Check for: 

• Spelling errors 
• Incorrect domain name 
• Extra spaces 
• Old or deactivated addresses 

Example: 

• Correct: [email protected] 
• Incorrect: name@domain,co.za 

If sending to a mailing list, remove invalid addresses. 

If the address is correct: 

• Check that the domain exists 
• Ensure it has valid MX records 

If the domain recently changed hosting, allow DNS propagation time. 

If you see: 

550 Relay denied 
550 Authentication required 

Verify: 

• SMTP authentication is enabled 
• You are using your full email address as the username 
• The password is correct 
• Port 465 (SSL) or 587 (TLS) is used 

Log in to: 

• cPanel → Email → Track Delivery 
• Plesk → Websites & Domains → Select your domain → Mail → Track Email Delivery 

Confirm whether the message was: 

• Rejected permanently (550) 
• Or deferred (temporary issue) 

A 550 error is typically a permanent rejection. 

What It Means: 
The recipient’s mailbox doesn’t exist or can’t receive mail. 

How to Fix It: 

• Double‑check the email address spelling 
• Confirm with the recipient 
• Try again later 

Why It Happens: Often simple typos or inactive accounts. 

What It Means: 
The recipient address is invalid. 

How to Fix It: 

• Correct any typos or spelling errors 
• Confirm the address with the recipient 

Request an alternative email address if needed 

What It Means: 
Your mail server isn’t authorised to send to this destination. 

How to Fix It: 

• Confirm your outgoing (SMTP) server settings (host, port, SSL/TLS) 
• Ensure SMTP authentication is enabled in your mail client 

Before You Move On: Check for public/unsecured networks, as they often block mail submission. 

What It Means:  
You hit a sending limit. 

How to Fix It: 

• Slow down the email rate 
• Use a dedicated email marketing service for bulk emails. 

What It Means:  
The recipient’s mailbox is full or restricted. 

How to Fix It: 

• Ask the recipient to clear space or adjust mailbox settings 
• Request an alternative contact address 

What It Means:  
Filters flagged your email. 

How to Fix It: 

• Remove suspicious links or attachments 
• Avoid trigger phrases or heavy formatting 
• Ask the recipient to whitelist your address 

What It Means: 
Recipient isn’t recognised on the mail system. 

How to Fix It: 

• Verify the email address 
• Ask the recipient to confirm the domain and directory settings 

What It Means: 
Your email or attachment is too large. 

How to Fix It: 

• Compress attachments 
• Use cloud file-sharing services 

What It Means: 
The email headers are incorrectly formatted. 

How to Fix It: 

• Manually re-enter the recipient’s email address 
• Check “From”, “To”, and “Subject” fields 
• Disable problematic mail client plugins 

Exim 

grep "550" /var/log/exim_mainlog 
tail -f /var/log/exim_mainlog 

Postfix 

grep "550" /var/log/mail.log
tail -f /var/log/mail.log

Goal: Identify recurring rejections, recipient domains, or specific sender issues. 

Exim 

• List queued messages: 

exim -bp       

• Remove stuck messages, if needed:

exim -Mrm <message-id>

Postfix 

• List queued messages:

postqueue -p 

• Delete stuck messages:

postsuper -d <queue-id>

Notes: 

• Verify rejected vs. deferred messages 
• Avoid mass deletion unless confirmed undeliverable 

Check SMTP authentication logs: 

/var/log/exim_mainlog or /var/log/maillog 

• Ensure SPF/DKIM/DMARC alignment 
• Run IP reputation checks to verify your server is not blacklisted: 
  • Verify SPF
  • Verify DKIM

dig +short TXT example.com 

dig +short TXT default._domainkey.example.com

swaks --to [email protected] --server mx.example.com --data "Subject: Test\n\nHello" 

telnet mx.example.com 25 

Goal: Confirm server accepts mail from authenticated user 

Yes, due to incorrect settings, lack of authentication, or reputation issues. 

No, it requires action before delivery will succeed. 

Good email hygiene and authentication reduce future issues.