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Introduction

Email communication relies on the Simple Mail Transfer Protocol (SMTP) to transfer messages between mail servers. During this process, several checks and validations are performed to ensure the email reaches the intended recipient.

However, if the receiving server encounters an issue such as authentication failures, policy restrictions, or mailbox problems, it cannot complete the delivery. In such cases, the sending server generates a Delivery Status Notification (DSN), also known as a bounce message.

A DSN provides detailed information about the delivery failure, helping users and administrators understand the reason the email was undelivered. 

This makes DSNs an essential part of diagnosing email delivery issues and maintaining healthy email communication.

Benefits of Understanding Email Bounces and Error Codes

• Helps you identify and diagnose the root cause of email delivery issues.
• Improves your overall email deliverability by ensuring messages reach the inbox.
• Detects authentication or configuration problems that may impact successful email sending.
• Maintains and protects your sender reputation by addressing issues proactively.
• Enables faster and more accurate troubleshooting when email errors occur.

In this article, you will learn about:

• Email Bounce categories
• Different Error codes

Key Terminology

 

Email Bounce Categories 

There are multiple bounce types that you can see in the report of a sent email. 

All bounce messages are primarily categorized as either Soft Bounce or Hard Bounce. A soft bounce is a temporary error. Once you fix it, the email can be delivered at a later time. A hard bounce is an error that prevents the email from being delivered. 

This section also covers additional bounce categories that, while not applicable to every bounce type, offer a simplified way to interpret bounce messages using industry best practices.

General bounce

This is treated as a soft bounce because we cannot determine the exact reason for delivery failure. Typically, this bounce type is associated with technical issues such as ‘Connection timed out’. We also classify a bounce as ‘general’ if the recipient server’s response is open to more than one interpretation. It could be a non-standard error message or too vague to be useful.

DNS failure

This bounce occurs when the recipient’s server is currently unable to deliver your email due to DNS issues on their end. This may or may not be a temporary problem. 

The error could be due to the mail server being down, or there was a typo when it was set up, or maybe the destination domain does not exist.

All we know is that the DNS host is unreachable. Therefore, we treat this as a soft bounce and allow some time for the problem to be rectified.

Mailbox full

The server cannot deliver your email because the recipient’s inbox is full. Most email applications have a set amount of storage that an individual user can use for emails. If this quota is exceeded, the server will not let any more mail through. It also alerts the mailbox owner to do something about it.

We treat this as a soft bounce if it is temporary, but we convert it to a hard bounce if the issue continues.

Note: This error can also indicate an abandoned mailbox. For example, someone might have set up a free webmail account just for shopping-related emails, then stops signing in if they have moved to a different location. 

Message too large

The size of your email, including headers, text, and images, is larger than the maximum size the recipient’s mailbox allows. The bounce message returned does not include information on the size limit, but we advise you not to send messages bigger than 99 KB.

Mail block - relay denied

Relay refers to the transmission of an email from one server to another. Your email has bounced due to a temporary error, which could be on the sending or the receiving side. 

This type of bounce usually occurs:

• When the sender’s message is not authenticated, or 
• Due to a misconfigured server on the recipient side.

Mail block - general

The recipient’s email server blocks inbound mail from the sender’s server, possibly due to blacklisting. A mail block is recorded when the receiving server completely blocks an email, rejecting it without attempting to deliver it to the inbox.

The reasons for this block are:

• Your reply-to address is blacklisted
• One of your sending IPs is temporarily blocked
• One of your sending domains is temporarily blacklisted
• The receiving server only accepts white-listed senders

Note: You can resend an email only if the above issues are resolved. Please contact [email protected] for more information.

Mail block - spam detected

The recipient’s email server has blocked your email because the content resembles spam email content. 

This mail block is often triggered because, 

• The recipient server is set up to detect spam emails 
• Your reply-to address or brand name has a low reputation 

We treat this as a soft bounce because some mail servers and email providers respond with false or incorrect error codes.

Transient bounce

The recipient's mail server can’t deliver your email to the inbox, but will keep trying for a limited time. We treat this as a soft bounce since there is a possibility the message will be delivered when the recipient's mail server retries.

Challenge or response bounce

The recipient has installed software as an anti-spam measure that only accepts email from previously authorized senders. If the software does not know the sender, a challenge email is returned, requiring a specific action before the original email is sent to the recipient.

Bounce - but no email address returned

The recipient mail server bounced your email, but did not indicate which address it was bouncing on behalf of. We determine the recipient based on the content of the bounce.

Hard bounce

Your email is permanently undeliverable to this email address because the address is either fake, was entered incorrectly, or the user's mailbox or domain is no longer active.

For example, the following are hard bounce error messages:

• Unknown or illegal alias
• Address rejected
• No such user here
• A bad destination mailbox address
• Rule imposed mailbox access for refused: user invalid

Different Error Codes

Here are a few standard SMTP error codes. We have divided them into Traditional Bounce Codes and Enhanced Bounce Codes for better understanding.

Traditional Bounce Codes

Traditional bounce codes are direct bounce codes that give a basic email error definition. Here are some traditional bounce codes:

 

Enhanced Bounce Codes

Enhanced bounce codes are advanced or more detailed email error messages that explain the SMTP error. Here are some enhanced bounce codes:

Note:

• Mail server administrators can customize messages that accompany bounce codes.
• If you need more information about a specific bounce in your report, please contact [email protected].​​​​​​​

Reference

• Improving Email Deliverability
• Avoid your emails from landing in Spam
• Authenticate Emails with SPF, DKIM, and SenderID

 

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