Mail Delivery Failed: Returning Message to Sender

​When you send an email and receive a “Mail delivery failed: returning message to sender” notification, it indicates that your message was not successfully delivered. This type of notification, known as a bounce or bounce-back message, is automatically generated by mail servers to inform senders of delivery issues.

The bounceback message will have this error in the subject, and the body should contain the original message attempting to be sent. If you know how to review these emails, they will typically tell you what the issue was. For example, the below bounce specifies:

The following address(es) failed:  [email protected]

We understand it can be challenging to review the entire contents of a returned email message at times, so we created the following tool to help you extract vital information.

• Bounceback Parser Tool

Example Mail Delivery Failed Message

For example, here is an email returned to the sender with the headers included.

Return-path: <>
Envelope-to: [email protected]
Delivery-date: Thu, 25 Oct 2012 16:42:54 -0400
Received: from mailnull by ecbiz103.inmotionhosting.com with local (Exim 4.77)
id 1TRUGT-0005Qd-RT
for [email protected]; Thu, 25 Oct 2012 16:42:54 -0400
X-Failed-Recipients: [email protected]
Auto-Submitted: auto-replied
From: Mail Delivery System <[email protected]>
To: [email protected]
Subject: Mail delivery failed: returning message to sender
Message-Id: <[email protected]>
Date: Thu, 25 Oct 2012 16:42:53 -0400</[email protected]></[email protected]> This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed: [email protected]
No Such User Here  ------ This is a copy of the message, including all the headers. ------ Return-path: <[email protected]>
Received: from [email protected] by ecbiz103.inmotionhosting.com with local (Exim 4.77)
(envelope-from <[email protected]>)
id 1TRUGT-0005QT-Q5
for [email protected]; Thu, 25 Oct 2012 16:42:53 -0400
To: [email protected]
Subject: [example.com] Please moderate: "About us"
X-PHP-Script: example.com/wp-comments-post.php for 123.123.123.123
Date: Thu, 25 Oct 2012 20:42:53 +0000
From: WordPress <[email protected]>
Message-ID: <[email protected]>
X-Priority: 3
X-Mailer: PHPMailer 5.1 (phpmailer.sourceforge.net)
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="UTF-8" A new comment on the post "About us" is waiting for your approval
https://example.com/ Author : Payday loans (IP: 123.123.123.123)
E-mail : [email protected]
URL : https://www.paydayspam.com
Comment: Payday loans are available for any occasion!

What Caused this Returned Message?

In the bounceback example above, the cause of the issue is that this user has a WordPress setup to allow comments on posts. A spammer attempted to leave a comment on one of their posts, and this site’s WordPress administrator has set up their e-mail account as [email protected]. So, that is the address used to send comment moderation requests. However, that e-mail address doesn’t exist on the RemoteDomain.com mail server, so it will always fail and cause a bounce-back error.

Typically, these bounceback errors are hard bounces, meaning trying to send the message again won’t be successful. You can read our previous article on Why Emails Bounce? for a more in-depth explanation of a hard bounce.

Common Causes of Mail Delivery Failure

Several factors can lead to email delivery failures:

• Invalid Recipient Address: The email address you entered may be incorrect or no longer in use. Even minor typographical errors can result in delivery failures.​
• Non-Existent Domain: If the domain part of the recipient’s email address (the portion after the ‘@’ symbol) doesn’t exist or is misspelt, the email cannot be routed correctly.​
• Recipient’s Mailbox Full: The recipient’s mailbox has reached its storage limit, preventing new messages from being accepted.​
• Spam Filters: The recipient’s mail server might flag your email as spam, leading to rejection or redirection to a spam folder.​
• Blacklisted Server: Your email server’s IP address could be blacklisted due to prior spam activities, causing recipient servers to reject emails from your server.​
• Attachment Size Limits: The email, including its attachments, exceeds the size limit set by the recipient’s email server.

Understanding Bounce Messages

Bounce messages provide specific information about why an email wasn’t delivered. Key components include:

• Error Codes: Numeric codes indicating the type of error. For example, ‘550’ typically means the mailbox is unavailable.
• Error Messages: Descriptions accompanying the error codes, such as “No Such User Here,” indicating the recipient address is invalid.

Reviewing these details can help identify the exact cause of delivery failure.

Steps to Resolve Delivery Failures

To address email delivery issues:

Verify Recipient Address: Ensure the email address is correct and active.​

1. Check Domain Validity: Confirm that the domain in the email address is valid and correctly spelled.​
2. Resend Later: If the recipient’s mailbox is full, wait and try sending the email again later.
3. Review Email Content: Avoid content that might trigger spam filters, such as excessive links or suspicious phrases.​
4. Monitor Server Reputation: Regularly check if your email server’s IP is blacklisted and take necessary actions to delist it.​
5. Reduce Attachment Size: Compress large attachments or use file-sharing services to send large files.

Since server issues can cause this error, asking the person you are sending it to can be helpful if they have any other related problems.

If you also have issues with WordPress comment spam causing bounce-back errors for your account, we have an advanced article on cleaning up WordPress comment spam that could help you resolve this issue.

Preventing Future Delivery Issues

To minimize future email delivery problems:

• Maintain a Clean Mailing List: Regularly update your contact list to remove invalid or inactive email addresses.​
• Authenticate Emails: Implement email authentication protocols like SPF, DKIM, and DMARC to enhance email deliverability and reduce the likelihood of your emails being marked as spam.​
• Monitor Bounce Rates: Keep an eye on bounce rates to promptly identify and address potential issues.

By understanding the causes of email delivery failures and taking proactive measures, you can improve the success rate of your email communications.