Introduction

Email messages are transferred between mail servers using a process called Simple Mail Transfer Protocol (SMTP). If there is an error during the transmission, a Delivery Status Notification (DSN), also known as a bounce message, is returned to the sending server explaining why the email was not delivered.

Different Bounce Categories 

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

All bounce messages are primarily categorized in either Soft Bounce or Hard Bounce. 

A soft bounce is a temporary error. Once you fix it, the email can be delivered after some time. A hard bounce is an error because of which the email can never be delivered. 

In this article, we have explained a few other bounce categories. These will not apply to all bounce types, but it is a simplified approach to interpret a bounce message based on the 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 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.

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, which may be due to blacklisting. A mail block is recorded when the receiving server blocks an email completely, rejecting the message without any attempt 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

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 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 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 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: