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Understanding Email Bounces and Error Codes

This article will help you understand the different reasons for email bounces and error codes.
R
Rashmi Kashyap
5 Feb, 2021 - Updated 2 months ago

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.

Note: This error can also be a sign of 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, 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

Note: You can resend an email only if the above issues are resolved. Please contact us at support@vtiger.com 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 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:

 

Code 

Type 

Explanation

420

Soft

Timeout connection problem: there were issues during the message transfer.

421

Soft

The recipient server temporarily deferred the message due to too many messages or too many connections in a short timeframe. 

422

Soft

The recipient’s mailbox has exceeded its storage limit.

431

Soft

Not enough space on the disk or an ‘out of memory’ condition due to a file overload.

432

Soft

Typical side-message: ‘The recipient’s exchange server’s incoming mail queue has been stopped’.

441

Soft

The recipient’s server is not responding.

442

Soft

The connection dropped during the transmission.

446

Soft

The maximum hop count has exceeded for the message: an internal loop has occurred.

447

Soft

Your outgoing message timed out because of issues concerning the incoming server.

449

Soft

A routing error - bad routing configuration or temporary routing issues, users may encounter an exchange server error.

450

Soft

Requested mail action not taken: mailbox unavailable. (For example, mailbox busy).

451

Soft

Requested action aborted: error in processing.

452

Soft

Requested action not taken: insufficient system storage.

471

Soft

An error in your mail server, often due to an issue with the local anti-spam filter.

500

Hard

The server could not recognize the command due to a syntax error.

501

Hard

A syntax error was encountered in command arguments.

502

Hard

This command is not implemented.

503

Hard

The server has encountered a bad sequence of commands.

504

Hard

The command parameter is not implemented.

510

Hard

Bad email address.

511

Hard

Bad email address.

512

Hard

A DNS error: the host server for the recipient’s domain name cannot be found.

513

Hard

‘Address type is incorrect’: another problem concerning address misspelling. In few cases, however, it’s related to an authentication issue

523

Hard

The total size of your mailing exceeds the recipient server’s limits.

530

Hard

Typically, an authentication problem,  but sometimes, it is about the recipient’s server blacklisting yours or an invalid email address.

541

Hard

The recipient address rejected your message: typically, it is an error caused by an anti-spam filter.

550

Hard

The user’s mailbox was unavailable (such as - not found).

551

Hard

The recipient is not local to the server.

552

Hard

The action was aborted due to exceeded storage allocation.

553

Hard

The command was aborted because the mailbox name is invalid.

554

Hard

The transaction failed for some unstated reason.

 

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:

 

Code 

Type 

Explanation

4.2.2

Soft

The email account is over quota

5.0.0

Hard

The address does not exist

5.1.0

Hard

Another address status

5.1.1

Hard

A bad destination mailbox address

5.1.2

Hard

A bad destination system address

5.1.3

Hard

Bad destination mailbox address syntax

5.1.4

Hard

Destination mailbox address ambiguous

5.1.5

Hard

Destination mailbox address valid

5.1.6

Hard

Mailbox has moved

5.1.7

Hard

Bad sender’s mailbox address syntax

5.1.8

Hard

Bad sender’s system address

5.2.0

Soft

Other or undefined mailbox status

5.2.1

Soft

Mailbox disabled, not accepting messages

5.2.2

Soft

Mailbox full

5.2.3

Hard

Message length exceeds the administrative limit

5.2.4

Hard

Mailing list expansion problem

5.3.0

Hard

Other or undefined mail system status

5.3.1

Soft

Mail system full

5.3.2

Hard

The system is not accepting network messages

5.3.3

Hard

The system is not capable of selected features

5.3.4

Hard

Message too big for the system

5.4.0

Hard

Other or undefined network or routing status

5.4.1

Hard

No answer from the host

5.4.2

Hard

Bad connection

5.4.3

Hard

Routing server failure

5.4.4

Hard

Unable to route

5.4.5

Soft

Network congestion

5.4.6

Hard

Routing loop detected

5.4.7

Hard

Delivery time expired

5.5.0

Hard

Other or undefined protocol status

5.5.1

Hard

Invalid command

5.5.2

Hard

Syntax error

5.5.3

Soft

Too many recipients

5.5.4

Hard

Invalid command arguments

5.5.5

Hard

Wrong protocol version

5.6.0

Hard

Other or undefined media error

5.6.1

Hard

Media not supported

5.6.2

Hard

The conversion required and prohibited

5.6.3

Hard

A conversion required but not supported

5.6.4

Hard

A conversion with loss performed

5.6.5

Hard

Conversion failed

5.7.0

Hard

Other or undefined security status

5.7.1

Hard

Delivery not authorized, message refused

5.7.2

Hard

Mailing list expansion prohibited

5.7.3

Hard

Security conversion required but not possible

5.7.4

Hard

Security features not supported

5.7.5

Hard

Cryptographic failure

5.7.6

Hard

The cryptographic algorithm not supported

5.7.7

Hard

Message integrity failure

9.1.1

Hard

Hard bounce with no bounce code found (invalid email or rejected email from your mail server such as -  sending limit)

 

Note

  • Mail server administrators can customize messages that accompany bounce codes.
  • If you need to know more about a specific bounce in your report, please contact us at support@vtiger.com. We will check your bounce logs for the exact DSN returned and investigate further.
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