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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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