Summary
The bounce message "Hard bounce - User Unknown [5.0.0 (SMTP reply matched bounce-rcpt pattern rule)]" indicates an email delivery failure where the recipient's mail server reported an issue. While the initial classification from the sender's Email Service Provider (ESP) may appear as a hard bounce for an unknown user, the underlying SMTP reply (the actual message from the receiving server) often reveals a different, more nuanced reason. This discrepancy is crucial, as a soft bounce, such as "mailbox full" (indicated by a 4.x.x status code), is temporary and can resolve itself, unlike a true hard bounce which signifies a permanent failure.
Key findings
- ESP classification vs. actual reply: The "Hard bounce - User Unknown" message is often an interpretation by your ESP, not the direct SMTP response from the recipient's mail server.
- SMTP reply code: The true SMTP bounce message, typically found in the DSN (Delivery Status Notification) diagnostic field, is what accurately explains the delivery failure. For example, a 452 4.2.2 indicates a mailbox is over quota, which is a soft bounce.
- Temporary vs. permanent: A 4.x.x status code denotes a temporary failure, meaning the email might be delivered later, or the issue (like a full mailbox) could be resolved by the recipient. A 5.x.x status typically indicates a permanent failure.
- Gmail-specific bounces: Authentic Gmail bounce messages typically end with -gsmtp. The ESP's mapped message might not reflect the actual Gmail response.
Key considerations
- Request raw SMTP logs: Always ask your ESP for the actual SMTP reply from the receiving server, not their categorized summary. This is essential for accurate troubleshooting.
- Address mail server settings: If the bounces are consistently 452 4.2.2 (mailbox full), these are temporary. You might consider temporarily pausing sends to these addresses to reduce noise and give the recipient time to clear their inbox, rather than immediately removing them from your list.
- Review ESP bounce handling: If your ESP is misclassifying soft bounces as hard bounces, this can lead to prematurely removing valid recipients from your list. Engage with their support to clarify their bounce handling rules and request adjustments if necessary.
- Monitor delivery metrics: Keep an eye on your Google Postmaster Tools and other postmaster dashboards for insights into your sending reputation and deliverability, as these can provide an independent view of how recipients' servers are reacting to your mail. More information about SMTP error codes can be found on SMTP Field Manual.
- DNS configuration: While less likely the primary cause for this specific bounce message (which points to mailbox issues), ensuring your DNS records (like SPF, DKIM, DMARC) are correctly configured is always a foundational step for good deliverability.
What email marketers say
Email marketers often encounter bounce messages that are difficult to decipher, particularly when the ESP's categorization differs from the raw SMTP reply. This can lead to confusion about the actual deliverability issue and impact list hygiene strategies. Marketers seek clarity on whether a bounce is temporary or permanent, and how to effectively communicate with their ESPs to get the precise information needed for troubleshooting.
Key opinions
- ESP interpretation issues: Many marketers believe their ESPs (like Pardot in this case) miscategorize bounce messages, leading to inaccurate assessments of email deliverability issues, such as labeling a soft bounce as a hard bounce.
- Desire for raw data: Marketers often express a need for the actual, raw SMTP bounce messages directly from the mail transfer agent (MTA) logs, rather than simplified or re-categorized summaries provided by their ESP.
- Impact on list hygiene: Misclassified bounces can lead to inadvertently removing active and engaged subscribers from mailing lists, which negatively impacts audience reach and engagement metrics.
- Frustration with support: There's a common sentiment among marketers of frustration when ESP support teams dismiss concerns about bounce miscategorization or insist on their internal classifications over the actual SMTP replies.
Key considerations
- Advocacy for accurate reporting: Marketers should persist in requesting the original SMTP bounce messages from their ESPs to get a clear picture of deliverability issues, especially for common recipient domains like Gmail.
- Handling temporary errors: For soft bounces like "mailbox full" (4.2.2), consider a temporary pause or a retry strategy instead of immediately treating them as permanent failures, to avoid losing valuable subscribers. Learn more about temporary bounces.
- DNS health as a preventative measure: While not directly tied to this specific bounce type, ensuring your domain's DNS is properly configured can prevent a range of other deliverability problems. Regular checks of your SPF records and other email authentication methods are always recommended.
- External resources for clarification: When ESP support is unhelpful, leverage authoritative external resources to understand SMTP error codes and bounce behavior independently. For instance, the Karol Cholewa bounce management guide provides examples of soft bounces.
Marketer view
Marketer from Email Geeks suggests that their ESP, Pardot, has been miscategorizing mailbox full soft bounces as hard bounces, despite no prior soft bounces for those addresses. This change in classification is causing concern as it affects list hygiene and subscriber engagement. They are trying to get the ESP to acknowledge this problem.
Marketer view
Marketer from Email Geeks explains they are new to email deliverability and are struggling to understand a hard bounce message from Gmail addresses, specifically "User Unknown [5.0.0 (SMTP reply matched bounce-rcpt pattern rule)]". They are seeing an increase in these bounces recently, which is causing concern regarding their email campaigns.
What the experts say
Email deliverability experts consistently highlight the critical distinction between an ESP's generalized bounce categorization and the precise SMTP reply from the recipient's mail server. They emphasize that the raw SMTP code and diagnostic message are the sole authoritative sources for understanding why an email bounced. Misinterpretation by an ESP can lead to incorrect list management decisions and impact overall sender reputation.
Key opinions
- Raw SMTP is paramount: Experts agree that the ESP's bounce classification (e.g., "Hard bounce - User Unknown") is often a simplified label, and the true reason for the bounce lies in the original SMTP response (the dsnDiag message from the delivering MTA).
- Misclassification is common: It's not surprising for ESPs to misclassify a temporary bounce (like mailbox full, 4.x.x) as a permanent hard bounce (5.0.0), which can be detrimental to sender lists.
- Google bounce identifiers: A genuine Google bounce message typically includes -gsmtp at the end, confirming it came directly from Google's servers.
- Temporary nature of over-quota: A 452 4.2.2 (over quota) bounce from Gmail is temporary. It does not automatically become a hard bounce, as users can clear space in their accounts, which are often shared across Google services.
Key considerations
- Insist on direct bounce data: Marketers should always request the raw, detailed bounce messages from their ESP's SMTP/MTA logs. This is the only way to get accurate, actionable insights for resolving deliverability issues.
- Implement intelligent retry/pause strategies: For temporary bounces, consider automated systems (possibly on the CDP side, before the ESP) that temporarily pause sending to affected addresses for a set period (e.g., 3 weeks) before retrying. This reduces unnecessary traffic and improves signal-to-noise ratio.
- Focus on deliverability signals: While preventing temporary bounces is good for operational efficiency, experts suggest that minor 4xy errors may not always warrant manual intervention or complex automation if the primary focus is reputation, as they are often self-correcting and generally don't impact IP or domain blacklists (or blocklists).
- DNS configuration: Although not the direct cause of a mailbox full bounce, misconfigured DNS can lead to various delivery failures. It's always a good practice to ensure your DNS records are impeccable.
Expert view
Expert from Email Geeks, Marcel, clarifies that the original bounce message is not from Gmail but from the ESP. He advises contacting the ESP to obtain the actual SMTP response or bounce message directly from their logs, as this is the only reliable way to understand the true reason for the bounce. ESPs' attempts to map or categorize these messages can often be unhelpful.
Expert view
Expert from SpamResource emphasizes that the proper handling of temporary failures, even for something as common as a mailbox being full, is crucial for maintaining sender reputation. Repeated attempts to a full mailbox can be seen as aggressive, even if they don't immediately result in a hard blacklist.
What the documentation says
SMTP documentation, notably RFCs, defines the meaning of various status codes that dictate how mail servers communicate delivery outcomes. The specific codes (e.g., 4.x.x for temporary failures and 5.x.x for permanent failures) are standardized. Understanding these allows senders to accurately interpret bounce messages, regardless of how an ESP might categorize them.
Key findings
- SMTP reply codes: The SMTP 452 reply, as seen in 452 4.2.2 (mailbox over quota), is a transient or temporary persistent negative completion reply. This means the command was not accepted, and the requested action could not be taken, but the condition is temporary.
- RFC compliance: According to RFCs, particularly RFC 3463, a 4.x.x status indicates a persistent transient failure. This means the message is temporarily deferred, and the sender should retry later. A 5.x.x status signifies a permanent failure, where no further attempts should be made.
- Diagnostic information: The diagnostic code (e.g., 4.2.2) provides more specific detail about the temporary failure, such as "mailbox full" or "system not accepting messages." This is what the dsnDiag field in DSNs should report.
- Gmail's specific handling: Google's support documentation confirms that 4.2.2 errors are indeed temporary and advise senders to inform the recipient to clear space, implying a non-permanent condition.
Key considerations
- Adhere to RFC standards: Senders and ESPs should process bounce messages based on the numerical SMTP reply codes and their standardized meanings, rather than relying solely on generalized descriptive text, to ensure proper list management and avoid unnecessary blacklisting (or blocklisting) of valid addresses.
- Implement retry logic: For 4.x.x errors, implementing appropriate retry mechanisms (e.g., retrying after a certain period) is the correct technical approach before considering an address permanently unreachable. This directly correlates with what happens when your IP gets blocklisted.
- Educate internal teams: Deliverability teams should ensure that customer support and technical personnel understand the difference between ESP-categorized bounce messages and the raw SMTP replies from receiving servers to provide accurate information to clients.
- Monitor dsnDiag values: When analyzing bounce data, prioritize the content of the dsnDiag field, as this contains the direct diagnostic message from the remote MTA, offering the most precise reason for the email's non-delivery.
Technical article
Documentation from RFC 3463, Section 3.3 states that a DSN 4.X.X status indicates a persistent transient error. This implies that the mail could not be delivered currently, but future attempts may succeed. The recipient mail system may also decide to keep retrying if appropriate, highlighting the non-permanent nature of these failures.
Technical article
Google's official support documentation for Gmail deliverability (specifically on 'over quota' issues) explains that a 4.2.2 error means the recipient's mailbox is full. They instruct senders to direct recipients to clear space, which confirms that this is a temporary, resolvable condition, not a permanent user unknown error.
Related resources
3 resources
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