Demystifying SMTP Error Messages: Understanding Bounce Codes and Their Meanings

Email communication has become an integral

part of our daily lives, whether for personal or professional use. However, despite its widespread use, sending and receiving emails isn't always seamless. Sometimes, emails fail to reach their intended recipients, and when this happens, it's crucial to understand why.

One of the key mechanisms for diagnosing email delivery issues is through SMTP error messages. SMTP, or Simple Mail Transfer Protocol, is the standard mail server to send and receive emails. When an email encounters an issue during transmission, the receiving server generates an SMTP error message, also known as a bounce message, and sends it back to the sender. These messages contain valuable information about why the email failed to be delivered.

Types of Bounce Messages

Bounce messages can generally be categorized as either soft bounces or hard bounces. A soft bounce is a temporary issue that prevents delivery, such as a full mailbox or a server timeout. On the other hand, a hard bounce indicates a permanent problem, such as an invalid email address or a blocked domain.

Let's delve deeper into some common types of bounce messages and what they signify:

1. General Bounce

A general bounce occurs when the reason for delivery failure is unclear or could be attributed to various technical issues. It's typically treated as a soft bounce, and resolving underlying technical issues can often lead to successful delivery on subsequent attempts.

2. DNS Failure

This bounce occurs when the recipient's server experiences DNS-related issues, making it temporarily unable to accept emails. While it's usually a soft bounce, allowing the recipient's server time to resolve the problem is essential.

3. Mailbox Full

If the recipient's mailbox is full and cannot accept any more messages, the sender will receive a mailbox full bounce notification. Initially treated as a soft bounce, repeated occurrences may escalate it to a hard bounce.

4. Message Too Large

When the email size exceeds the recipient's mailbox limit, it results in a bounce message indicating that the message size is too large for delivery.

5. Mail Block - Relay Denied

This bounce occurs due to a temporary error in relaying the email, often caused by authentication issues or misconfigured

servers. It's crucial to address the underlying cause to ensure successful delivery.

6. Mail Block - Spam Detected

If the recipient's server detects that the email resembles spam content, it may block the email from being delivered. While treated as a soft bounce, reviewing the email content and the sender's reputation is essential to prevent future blocks.

7. Transient Bounce

In a transient bounce, the recipient's mail server cannot deliver the email immediately but will continue to retry for a limited time. It's treated as a soft bounce, with the potential for successful delivery upon subsequent attempts.

8. Challenge or Response Bounce

Some recipients employ anti-spam measures that require senders to complete a specific action before the email is delivered. These challenge or response bounces necessitate sender interaction for successful delivery.

9. Hard Bounce

A hard bounce indicates a permanent delivery failure, often due to an invalid email address, inactive mailbox, or blocked domain. Removing hard-bounced addresses from your mailing list to maintain deliverability is essential.

Understanding Error Codes

SMTP error messages are accompanied by error codes that provide additional insight into the cause of the delivery failure. These codes follow a specific format and are divided into traditional and enhanced bounce codes.

Traditional Bounce Codes

Traditional bounce codes offer basic email error definitions, including 421 (temporary failure) and 550 (permanent failure). These codes help identify issues like mailbox unavailability or syntax errors.

Enhanced Bounce Codes

Enhanced bounce codes provide more detailed error messages, such as 5.1.0 (another address status) or 5.7.1 (delivery not authorized). These codes offer greater specificity, aiding in diagnosing and resolving delivery issues more effectively.

Conclusion

SMTP error messages play a crucial role in diagnosing email delivery issues, helping senders understand why their emails failed to reach their intended recipients. By familiarizing themselves with common bounce types and error codes, senders can take appropriate actions to address underlying issues and improve email deliverability. Remember, resolving delivery issues ensures successful communication and helps maintain the sender's reputation and trustworthiness in the digital landscape. Email Error Codes In Depth Explained

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.
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)
5.1.10	Hard	RESOLVER.ADR.RecipientNotFound; Recipient rebaz.jalizada@se.mcd.com not found by SMTP address lookup
5.4.316	Soft	Message expired, connection refused
4.2.1	Hard	mailbox temporarily disabled
4.4.2	Hard	The email account that you tried to reach does not exist.

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