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Common Error Messages for Returned Email

Details

You may receive a returned message error when trying to send an email. The table below explains the reasons that an email may be returned. The exact error number and message may vary by IP address and domain name.

Most Common Returned Message Reasons

Use the table below to view common returned email error messages.

Note: For other error codes that a sender may encounter, refer to postmaster.cox.net.

Error Number Error Message Error Reason
450 Recipient Address Rejected: Domain Not Found

To protect our email system from being flooded by email list administrators, or by spammers, we have instituted rules to manage incoming emails to our system. These rules are based on common industry practices on how to send large amounts of email without causing potential harm to the recipient's mail server.

  • Occasionally the mailing list you referred to in your message to us is soft bounced by our servers, because the mailing it was part of, exceeded one or more of our rules for incoming email.
  • A soft bounce means that we will deliver the email in question, although it will result in a short delay in receipt.
  • We are working with various email list administrators and other large websites to ensure that they conform to reasonable methods of delivering bulk mailings when generating large mailings to our users. We regret any inconvenience these soft bounces may cause, now or in the future.
  • It is also possible that the mail servers that were responsible for delivering the message to us have been blocked due to a high number of concurrent connections.
    • This type of abuse is usually indicative of a spammer trying to send unsolicited commercial emails.
    • The message might have been blocked along with all other messages from that mail server. Since the email provider in question has been added to our block list, this bounced message will continue to be generated.
  • Contact technical support for further assistance with this issue.
500 Recipient Address Rejected: Recipient Mailbox is Full

The Cox email address in question is over its storage limit. Cox email accounts are currently set up with a 30 MB storage limit.

  • If a user of Cox goes over the 30 MB storage limit, emails sent to that account will be returned to the sender until the space has been cleared.
  • We have no way of knowing if the user in question is aware of the status of their storage space, or the frequency with which they check their quota.
503 Improper Use of SMTP Command Pipelining The message was sent in a non-conventional, non-standard way. Have the sender of this email contact us if they have further questions.
540 Recipient Address Rejected: Suspension If this is in regards to your Cox email address, you will need to call and speak with a Cox Business Billing Representative at 866-272-5777 in order to have your email address re-activated.

If you received this error regarding another Internet Service Provider's (ISP) email account, you will need to contact the individual to have them contact their ISP for assistance.
550 User Unknown If the Cox email account in question is generating returned messages that state "User Unknown" when an individual attempts to send an email to it, it could mean that the email account was not activated after the user signed up for their user account.

If you are sure that the Cox email account in question has been fully activated, and the account in question is still generating "User Unknown" bounced messages, please contact Cox Business Internet Technical Support at 866-272-5777 for further assistance.
550 Client Host Rejected This can occur when the mail servers that were responsible for delivering the message to us were temporarily blocked due to a high number of concurrent connections. This type of abuse is usually indicative of a spammer trying to send unsolicited commercial emails. Therefore, the message might have been blocked along with all other messages from that mail server at that time. However, it is likely that the block was subsequently removed.
554 Client Host Rejected: Access Denied Your message is being blocked by our site because it either had characteristics similar to spam (unsolicited commercial email), or you're sending your message from an Internet Service Provider (ISP) or email provider who has been identified as having allowed spam to be generated from their facilities. For the protection of our users, we do not deliver bulk, unsolicited emails to Cox email accounts.

If you do require the ability to perform mass mailing, please refer to massmailing.coxmail.com for approval requests.

If your Internet Service Provider (ISP) or email provider has been added to our block list, your email message will be rejected, or a bounced message will continue to be generated. If this is the case, please contact your email administrator and ask them to contact us for further assistance.
554 Sender Address Rejected This address is listed on our black list of email addresses, or is on the black list of one of the spam filter lists we use to lessen the amount of spam our users receive. You should have the owner of this address contact us if they have questions.
554 Service Unavailable The message in question was blocked because it came from a domain or IP address that is currently listed with one of our spam filters. These filters are utilized to help prevent excessive spamming to our email providers. If your domain or IP address is listed with any of the spam filters we currently use, your message will not be delivered.

If customers believe they have been unfairly added to any or all of these particular lists, they may contact the website in question.

spamhaus (www.spamhaus.org)
554 Relay Access Denied This error means that the person sending the email was not authorized to use the email server (SMTP) to send email. They should contact their Internet Service Provider (ISP) or email provider. If you are a Cox Business Internet customer, please call Cox Business Technical Support at 866-272-5777 for Relay Server Information.
554 Relay Access Denied: Header Subject This subject line is on our black list of subject lines, or is on the black list of one of the spam filter lists we use to lessen the amount of spam our users receive. You should have the sender of this email contact us if they have questions, or ask them to change the subject line, which unfortunately appears to have been used as the subject of a spam message at some time.
CXBL Sending IP Address Blocked

The sending IP address has been blocked by COX due to exhibiting spam-like behaviour.

Send an email request to unblock.request@cox.net and include the following information:

  • First name
  • Last name
  • Email address
  • Phone number
  • Domain
  • IP/CIDR range of outgoing email servers
  • Detailed description and the corrective action that has been taken
  • Email server logs related to the issue

    Note: Cox has sole discretion on whether to unblock the sending IP address.
CXTHRT Email Sending Limited Due to Suspicious Activity

To secure your account, reset your password at cox.com/password. It may take up to 2 hours to confirm security and remove sending limits.