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Understanding Email Bounces and Error Codes
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.
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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
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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:
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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:
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