Summary
A 'hard bounce user unknown' error, indicated by a 5.0.0 SMTP code, signifies a permanent delivery failure. This indicates the receiving email server encountered a critical error, deeming the email undeliverable, and future attempts will likely fail. The 'User Unknown' component typically implies the email address is invalid, doesn't exist at the recipient's domain, the user account has been closed, or recipients marked the sender as unwanted. Possible reasons include typos, domain issues, the recipient server blocking delivery, or the domain not existing anymore. DSN codes starting with '5' confirm permanent failure. Real Google bounces end with '-gsmtp', absence indicates a non-genuine bounce. ESPs might misclassify bounces, especially regarding temporary issues like over quota (signaled by a 452 code) requiring careful interpretation. Maintaining good list hygiene by promptly removing hard bounces is crucial for sender reputation and deliverability, involving proactive list management and potentially email validation services.
Key findings
- Permanent Failure: The 5.0.0 SMTP code signifies a permanent delivery failure, indicating the email will not be delivered.
- Invalid Recipient: "User Unknown" suggests the email address is invalid or doesn't exist at the recipient's domain, or the user account has been closed.
- List Hygiene: Removing hard bounces from your email list is critical for maintaining a good sender reputation and improving email deliverability.
- Google Bounce Verification: Real Google bounces end with '-gsmtp'; lack of this indicates a non-genuine bounce.
- Misclassification Potential: ESPs can misclassify bounces, necessitating careful interpretation, particularly with temporary issues like over quota situations.
- Sender Reputation Impact: Repeatedly sending to invalid addresses negatively impacts sender reputation, potentially leading to being flagged as a spammer.
- Other reasons for User Unknown: Recipients might mark senders as unwanted, causing hard bounces
Key considerations
- ESP Response: Consult your Email Service Provider (ESP) to understand the specific bounce message and SMTP response for accurate diagnosis, including clarifying potential misclassifications.
- Monitor and Remove: Implement a system for monitoring bounces and automatically removing hard bounced addresses from your mailing list to prevent future sends.
- List Validation: Consider using email validation services to proactively identify and correct invalid email addresses before sending, minimizing hard bounces.
- Google Bounce Validation: Verify if bounces from Gmail addresses truly originate from Google (check for '-gsmtp') to avoid incorrect classification.
- Over Quota Monitoring: Monitor over quota bounces, understanding these can be temporary (452 code), and avoid prematurely removing valid subscribers.
- Proactive List Management: Implement proactive list management practices, including regular cleaning and validation, to maintain a healthy subscriber list.
What email marketers say
10 marketer opinions
A 'hard bounce user unknown' error, indicated by a 5.0.0 SMTP code, signifies a permanent delivery failure, meaning the email could not be delivered and further attempts are unlikely to succeed. The 'User Unknown' component typically implies the email address is invalid, doesn't exist at the recipient's domain, or the user account has been closed. Possible reasons include typos, domain issues, or the recipient server blocking delivery. Maintaining good list hygiene by promptly removing hard bounces is essential for sender reputation and deliverability.
Key opinions
- Permanent Failure: The 5.0.0 SMTP code signifies a permanent delivery failure, indicating the email will not be delivered.
- Invalid Recipient: "User Unknown" suggests the email address is invalid or doesn't exist at the recipient's domain.
- List Hygiene: Removing hard bounces from your email list is critical for maintaining a good sender reputation and improving email deliverability.
- DSN Interpretation: The 'dsnstatus' may be a made up response and it may be wise to request the actual SMTP response from ESP.
- Root cause: Typo in the recipient's address or the user may have left the company.
Key considerations
- ESP Response: Consult your Email Service Provider (ESP) to understand the specific bounce message and SMTP response for accurate diagnosis.
- Monitor and Remove: Implement a system for monitoring bounces and automatically removing hard bounced addresses from your mailing list.
- List Validation: Consider using email validation services to proactively identify and correct invalid email addresses before sending.
- Bounce Misclassification: It may be wise to double check that your ESP is classifying the correct type of bounce message and dsn status response.
Marketer view
Email marketer from Reddit shares that a hard bounce often means the email address is no longer valid, the domain might not exist anymore, or the recipient's server has blocked the sender. They recommend immediately removing these addresses from the mailing list.
10 Feb 2023 - Reddit
Marketer view
Email marketer from Email Geeks explains that the bounce message is from the ESP and the actual SMTP response should be requested from them to understand the real reason for the bounce.
14 Aug 2023 - Email Geeks
What the experts say
6 expert opinions
A hard bounce with the 'user unknown' error, often indicated by a 5.0.0 code, signifies that the recipient's email address is invalid or non-existent. This can occur because the address never existed, the account was closed, or recipients marked the sender as unwanted. Real Google bounces end with '-gsmtp'; absence indicates a non-genuine bounce. ESPs might misclassify bounces, as seen with Pardot's bounce handling rules. Over quota situations may also trigger bounces, but these can be temporary, signaled by a 452 code. Effective list management, proactive removal of invalid addresses, and proper bounce interpretation are crucial for maintaining sender reputation and deliverability.
Key opinions
- Invalid Address: 'User Unknown' signifies an invalid or non-existent email address.
- Unwanted Sender: Recipients might mark senders as unwanted, causing hard bounces.
- Google Bounce Verification: Real Google bounces end with '-gsmtp'; lack of this indicates a non-genuine bounce.
- Misclassification Potential: ESPs can misclassify bounces, necessitating careful interpretation.
- Over Quota Handling: Over quota bounces may be temporary (signaled by 452 code); ensure subscribers aren't removed prematurely.
- List Cleaning Importance: Regular list cleaning by removing hard bounces is essential to avoid being flagged as spam.
Key considerations
- Bounce Interpretation: Understand the specific bounce messages and codes provided by your ESP.
- Google Bounce Validation: Verify if bounces from Gmail addresses truly originate from Google (check for '-gsmtp').
- Over Quota Monitoring: Monitor over quota bounces to ensure temporary issues don't lead to permanent removal of valid subscribers.
- Proactive List Management: Implement proactive measures to manage your email list and keep email addresses clean.
- ESP Configuration: Ensure correct ESP configuration of bounce classifications to avoid inaccurate hard bounce classifications
Expert view
Expert from Email Geeks clarifies that over quota bounces at Google tend to be temporary (signaled by a 452 code) and suggests ensuring that the subscriber isn't removed from future sends due to the misclassification. Ideally, monitor the address for subsequent sends to confirm the quota issue is resolved.
26 Dec 2023 - Email Geeks
Expert view
Expert from Email Geeks shares that real Google bounces end with "-gsmtp", and if that's missing, it's not a genuine Google bounce.
12 Dec 2024 - Email Geeks
What the documentation says
5 technical articles
A hard bounce, characterized by a 5.0.0 SMTP status code, signifies a permanent delivery failure. This indicates the receiving email server encountered a critical error, deeming the email undeliverable, and future attempts will likely fail. DSN codes starting with '5' confirm permanent failure. A common cause, represented by 'User Unknown', means the recipient address is invalid, non-existent, or rejected. The recommended action is to immediately cease sending emails to such addresses to maintain deliverability.
Key findings
- Permanent Failure: A 5.0.0 SMTP status code indicates a permanent email delivery failure.
- Invalid Address: "User Unknown" implies the recipient's email address is invalid or does not exist.
- DSN Code '5': DSN codes starting with '5' signify a permanent error and undeliverable email.
- Cessation of Sending: It is advised to immediately stop sending emails to addresses that generate a hard bounce due to an invalid recipient.
Key considerations
- Address Removal: Immediately remove hard bounced email addresses from your mailing lists.
- List Validation: Implement email list validation to identify and remove invalid addresses proactively.
- Bounce Monitoring: Continuously monitor bounce rates to identify potential deliverability issues.
Technical article
Documentation from RFC 3463 explains that the first digit '5' of a DSN (Delivery Status Notification) code indicates a permanent failure. The second digit indicates the class of failure, and the third provides detail. A 'User Unknown' error falls under this permanent failure category.
22 Jun 2024 - RFC 3463
Technical article
Documentation from Oracle explains that a hard bounce is a permanent failure to deliver email, generally caused by a non-existent or invalid email address. It is important to remove these from your email list as the email is undeliverable.
20 Mar 2022 - Oracle
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