Someone else is receiving a bounceback message when sending to me

Common Bounce Messages Explained

SMTP Responses

SMTP responses provide a brief explanation of why an email message was not delivered. Response codes, subcodes, and messages vary among mail providers, but must comply with relevant RFC specifications. Response codes for failed delivery fall into two categories: Deferred (4xx) and Rejected (5xx). Deferral and Rejection codes indicate temporary and permanent issues respectively that are preventing delivery to one or more recipients.


A 4xx error indicates a temporary failure, which means that delivery cannot be completed at this time. In the event of a deferral, the sender should retry delivery later. An example of a temporary failure would be a mailbox receiving too much mail in a short period of time, causing some mail to be deferred for later delivery.


A 5xx error indicates a permanent failure. This error means there is a problem with the message content, formatting, or recipients that prevents delivery. These issues must be addressed before delivery can be re-attempted by resending the message. In most cases, the message contained in the SMTP response will indicate the specific problem with the message. An example of a permanent failure is a message sent to a recipient that does not exist.

Below is a list of common bounce messages. You'll find an explanation and solution, if applicable

400 - Temporary Failures

450 Requested mail action not taken: mailbox unavailable

  • Explanation: The recipient mailbox was too busy to handle the request.
  • Solution: Wait, the server will retry again later.

450 4.1.8 <>: Sender address rejected: Domain not found

  • Explanation: Domain doesn't exist, no valid DNS or A Record for this domain
  • Solution: Create appropriate DNS records for the domain

451 4.3.0 <servername[xx.xx.xx.xx]>: Client host rejected: Throttled - Too much spam from your mail server. Try again later

  • Explanation: Too much spam from that server
  • Solution: Stop spamming from that server. Resend later. Whitelist IP.

451 4.3.0 <>: Temporary lookup failure

  • Explanation: Known issue that our engineers are currently working to resolve. Rarely occurs.
  • Solution: Resend the message

451 4.3.5 <>: Sender address rejected: Server configuration error

  • Explanation: Known issue that our engineers are currently working to resolve. Rarely occurs.
  • Solution: Resend message

452 4.7.0 Too many connections

  • Explanation: The sender's mail server has too many concurrent connections with our server.
  • Solution: Have the sender's mail server or provider reduce the number of concurrent connections that they make to our servers to five or less.

500 - Permanent Failures

504 5.5.2 <user@nonqualifieddomain>: Sender address rejected: need fully-qualified address

  • Explanation: The sender is using a domain that cannot possibly be a valid domain
  • Solution: The sender should use a real domain name

550 5.0.0 Your message has been rejected because you have been detected sending spam

  • Explanation: The content of your messages was flagged as spam. 
  • Solution: Try sending a different message to see if it will go through. If so, you may need to review your original message content and ensure it does not contain any spammy content.

550 5.0.0 Your message has been rejected because you have been detected sending a virus

  • Explanation: This particular message was scanned and it was determined that it contained a virus. We have a zero-tolerance policy for sending viruses through the mail system
  • Solution: Remove any attachments and ensure the message does not contain a virus. You may also want to do a virus scan on your computer in case it has been infected.

550 5.0.0 Your message has been rejected because you have exceeded sending limits

  • Explanation: Your mailbox has sent out too many messages and violated our outbound bulk mail policy
  • Solution: You will need to wait until your daily sending threshold has dropped below the limit to send external mail. 

550 5.1.0 <>: Sender address rejected: User unknown in relay recipient table

  • Explanation: The sender's domain exists in the hosting system, but the sender's full email address does not    
  • Solution: Add email address (mailbox, alias, group) to the hosted domain or delete hosted domain

550 5.1.1 <>: Recipient address rejected: User unknown in relay recipient table

  • Explanation: The recipient's email address doesn't exist in the hosted domain    
  • Solution: Add email address to a hosted domain

550 5.4.1 <>: 550 5.4.1 recipient address rejected access denied

  • Explanation: This error message indicates there is a domain configuration issue or DNS is not configured correctly.
  • Solution: Check the MX records and other DNS records for the domain name. If you are using Office 365 or Exchange Online, check the admin console to ensure DNS it correctly set up. This error is usually related to Exchange services with Directory Based Edge Blocking (DBEB).

550 5.7.1 <>: Relay access denied

  • Explanation: This error usually indicates that the mailbox does not exist in our environment 
  • Solution: Double-check the address and make sure it is valid. You may also need to look for any typos, spaces, or additional punctuation marks (like quotes(') or double quotes(")) in the address

554 5.7.1 <servername[XX.XX.XX.XX]>: Client host rejected: The sender's mail server is blocked

  • Explanation: Sending IP is blocked internally    
  • Solution: Whitelist IP and/or sending domain/address to begin receiving email from the sender. 

554 5.7.1 ACL dns_rbl; Client host [XX.XX.XX.XX] blocked using"

  • Explanation: The sending server is on a real-time blocklist     
  • Solution: Whitelist the blocklisted IP

554 5.7.1 <>: Sender address rejected: Blocked by this domain

  • Explanation: The sender is blacklisted at the domain level. 
  • Solution: Remove the sender from the domain level blacklist

554 5.7.1 <>: Sender address rejected: Access denied

  • Explanation: Either the sender's user name/password is wrong, authentication is not on in the mail client
  • Solution: Have sender authenticate with the correct username (full email address) & password on SMTP server and verify that SMTP authorization is turned on in the mail client. You can use the [Setup Tool] to ensure all of your settings are correct for that mail client

554 5.7.1 <>: Sender address rejected: ACL recipient_nonull - Too many bounces to this recipient

  • Explanation: This message is a bounce and we're not receiving bounces for messages originating from other servers    
  • Solution: Have the mail client use a return path address that is not hosted in our system

554 5.7.1 <>: : Sender address rejected: ACL from_senders_regexp

  • Explanation: The sender failed one of our regular expression filters. Most often because of the addition of "bounce." in the beginning of their domain    
  • Solution: Safelist the sending address in the active directory and monitor delivery reports.

554 5.7.1 Sender address rejected: ACL mx_access mail server in RFC 1918 private network (in reply to RCPT TO command)

  • Explanation: The domain that mail is being sent from has MX records but the IP address that the MX records point to is on a Private Network (10, 192.168.x.x, 172.16.x.x). 
  • Solution: change the MX records to point to valid internet routable IP addresses or change the address you are sending from so it is coming from a valid domain

554 5.7.1 <> Client host rejected: You are not allowed to send mail to this recipient.

  • Explanation: This bounce occurs when exclusive spam filtering is turned on and a user who is:Solution: Safelist the sender's address or change spam filtering preferences from "Exclusive" to "On" to allow for incoming mail flow to continue. 
    • 1) Not on the recipient's domain 
    • 2) Is not safelisted by the domain / user tries to send to a specific user

554 5.5.2 <user@nonqualifieddomain>:Invalid data in message> #SMTP#

  • Explanation: The problem is when an attached document name is longer than 50 characters.
  • Solution: Shorten the name of the document you are sending.
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