Summary
Soft bounce errors 4003 and 4004 from Gmail are specific, non-standard codes that often indicate temporary technical issues rather than content or reputation problems. These errors typically point to transient network problems, such as a server being down, routing issues, or DNS errors on the recipient's side. Unlike hard bounces that signify permanent failures, soft bounces suggest a temporary impediment to delivery, implying that reattempting delivery might succeed.
Key findings
- Non-standard codes: Gmail's 4003 and 4004 are not official SMTP 4xx bounce codes. They are internal error codes generated by the ESP.
- Technical rather than reputation: These errors are generally not related to your sender reputation or email content. They typically indicate a technical glitch.
- Common causes: 4003 often signals server downtime or routing issues, while 4004 may indicate DNS errors, or internal or external network problems.
- Temporary nature: As soft bounces, these are temporary failures. The email server instructs the sending server to retry delivery later. You can learn more about the distinction between hard and soft bounces from SendLayer.
- Shared IP pools: If using a shared IP pool with an ESP, a problem affecting the pool's routing or connectivity can lead to widespread 4003/4004 errors for multiple senders.
Key considerations
- Check ESP diagnostics: Always consult your Email Service Provider's (ESP) documentation for their specific interpretation of these codes. They may have internal meanings for errors like sudden spikes in Gmail bounces.
- Verify ESP status: Check your ESP's status page or contact their support. Large volume soft bounces often point to an issue on their end that impacts delivery to Gmail, rather than your sender practices.
- Retry strategy: Since these are temporary errors, ESPs typically automatically retry sending. If a significant volume of emails consistently fails with these codes, consider a strategic resend after a period of time.
- Monitor deliverability: While not reputation-related directly, consistent issues can impact overall deliverability. Keep an eye on your Gmail Postmaster Tools data for any broader trends, though these specific error codes won't be visible there.
What email marketers say
Email marketers often encounter frustration with obscure bounce codes like Gmail's 4003 and 4004. Their primary concern is usually whether these errors signify a problem with their content, list hygiene, or sender reputation. They typically lack direct access to the detailed SMTP logs needed for precise diagnosis, relying heavily on their ESPs for interpretation and resolution. Marketers generally seek confirmation that these are technical, non-reputation-damaging issues that can be overcome through automatic retries.
Key opinions
- ESP transparency: Many marketers express a desire for greater transparency from ESPs regarding detailed bounce messages and diagnostic information, highlighting the difficulty in troubleshooting when such data is hidden.
- Reputation concern: The immediate worry for many marketers when seeing a spike in bounces, even soft ones, is whether it indicates a problem with their sender reputation or if their emails are perceived as spam.
- Shared IP pool issues: Marketers recognize that if they're on a shared IP pool, issues can arise that affect multiple clients simultaneously, leading to widespread soft bounce problems to specific ISPs like Gmail.
- Focus on fundraising emails: Some marketers note that certain types of emails, such as fundraising appeals, might be particularly susceptible to temporary delivery glitches during high-volume sending periods.
Key considerations
- ESP communication: Marketers should actively engage with their ESP's support team when encountering unexplained soft bounce spikes to get clearer answers and ensure the issue is addressed. This is key for understanding higher soft bounce rates for Gmail.
- Bounce reporting accuracy: It's important to understand how an ESP translates underlying SMTP codes into their user-facing bounce messages, as this can vary widely. A basic understanding of common bounce reasons is helpful.
- Focus on deliverability best practices: Despite these being technical errors, maintaining a strong sender reputation through good list hygiene and engagement practices remains crucial for overall deliverability.
- Automated retries: Since these are temporary, rely on your ESP's automated retry mechanisms. Manual intervention is typically not required unless issues persist for extended periods.
Marketer view
An Email marketer from Email Geeks explains their client's experience: they had three clients recently see a significant spike in soft bounces from Gmail, affecting 20-70% of their list. These clients are small-volume, B2C nonprofits sending through the same ESP with shared IPs, yet they maintain good Google Postmaster Tools reputation.
Marketer view
An Email marketer from Email Geeks states that all their affected clients use the same ESP, suggesting a potential issue related to the ESP's infrastructure, possibly even the same IP pool.
What the experts say
Deliverability experts often emphasize the critical importance of understanding the exact bounce message from the receiving server, as proprietary codes like Gmail's 4003 and 4004 can be misleading. They point out that these non-standard errors typically suggest underlying network or infrastructure issues that prevent successful message delivery rather than content-related spam filtering. Experts strongly advocate for ESPs to provide more transparent, standard SMTP response codes to empower senders in troubleshooting.
Key opinions
- Importance of specific messages: Experts stress that the actual bounce message from Gmail is paramount for diagnosis, as it contains the definitive reason for non-delivery.
- Non-legal SMTP codes: True deliverability experts quickly identify 4003 and 4004 as non-standard, proprietary codes, indicating that the ESP is abstracting the real SMTP responses.
- Temporary networking issues: The consensus among experts is that these codes typically signify temporary network problems, such as an inability to reach receiving servers, routing glitches, or DNS resolution failures. This aligns with findings on domain does not exist errors.
- Not spam related: Crucially, experts confirm that these specific soft bounces are unlikely to be related to spam filtering or sender reputation issues.
Key considerations
- Demand better diagnostics: Senders should press their ESPs for access to the raw SMTP bounce messages or more detailed, standard-compliant explanations of these errors. Without this, effective email deliverability troubleshooting is hindered.
- Rely on resends: Given their temporary nature, the primary action for senders is to allow the ESP to automatically retry sending the messages. This is the intended behavior for soft bounces.
- Monitor broader trends: While 4003/4004 errors are isolated, a persistent high volume could indicate systemic network instability on the ESP's side. Monitoring overall deliverability metrics in tools like Google Postmaster Tools is still advisable.
- Assess ESP's infrastructure: If these non-standard errors are frequent, it might signal underlying routing or server issues with the ESP that need to be addressed by them directly.
Expert view
A Deliverability expert from Email Geeks inquires about the specific bounce message from Gmail, emphasizing its importance over the sending message for proper diagnosis.
Expert view
A Deliverability expert from Email Geeks also asks where in the email transaction the bounce occurred, indicating the need for detailed log information often hidden by ESPs.
What the documentation says
Formal email documentation, such as RFCs, defines standard SMTP response codes but does not explicitly detail proprietary codes like Gmail's 4003 and 4004. These non-standard errors typically fall under the broad category of 'temporary failure conditions' (4xx range). The documentation generally emphasizes that 4xx errors mean the client (sending server) should retry delivery later, as the condition is expected to be temporary. Understanding these nuances is vital for proper email system configuration and deliverability best practices.
Key findings
- RFC 5321 defines 4xx: The foundational SMTP RFC 5321 specifies that 4xx codes denote a temporary negative completion reply, indicating that a command was not accepted for a transient reason, and the sending system should retry.
- Proprietary codes: Error codes like 4003 and 4004 are often internal or proprietary to specific Email Service Providers or Mail Transfer Agents (MTAs), acting as wrappers for more standard underlying issues.
- Network and server issues: Documentation often attributes general 4xx errors to transient network problems, server outages, or temporary resource unavailability on the recipient's side, consistent with common soft bounce causes.
- Sender responsibility: While recipient-side issues, senders are still responsible for managing bounce rates and ensuring their systems can handle retries effectively.
Key considerations
- Interpret proprietary codes: While not standard, it's crucial to consult ESP-specific documentation to understand what their proprietary codes (like 4003, 4004) signify within their system, relating them to standard SMTP contexts.
- Rely on retry logic: Mail systems are designed to retry 4xx errors. Senders should ensure their ESP's (or their own MTA's) retry schedules are configured appropriately to overcome temporary delivery issues.
- Differentiate from hard bounces: It's important to properly categorize soft bounces from hard bounces to avoid unnecessarily removing valid recipients from mailing lists.
- Review network configuration: Ensure your own email infrastructure (or your ESP's) has robust DNS resolution and network connectivity to minimize errors that could lead to 4003/4004 issues.
Technical article
Email documentation from RFC 5321 (SMTP) explains that transient negative completion replies, denoted by 4xx codes, signify a temporary failure condition where the command was not accepted, and the sending client is encouraged to try again later, differentiating them from permanent failures.
Technical article
Google Postmaster Tools Help indicates that while specific internal error codes like 4003 and 4004 are not directly listed in their public dashboards, senders can observe broader patterns in delivery rates and spam reports that might correlate with underlying transient issues.
Related resources
1 resources
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