What causes 451 4.3.2 errors when sending email to Netease in China?

Key findings

• Temporary nature: A 451 error generally indicates a temporary problem that prevents immediate delivery, not a permanent rejection. It's often related to server availability or resource constraints.
• Netease's unique behavior: Netease is known for its stringent and often unconventional email filtering policies. New senders typically start with low reputation and must actively build it through consistent good sending practices and sufficient volume.
• Content and message factors: Issues like content encoding, language, message size, or attachment types can trigger these errors, especially if the rejection occurs late in the SMTP conversation (e.g., after the DATA command).
• Combination of factors: Persistent 451 errors could stem from a combination of low sender reputation and specific internal issues or bugs on Netease's side, which might result in generic error messages.

Key considerations

• Analyze bounce patterns: Distinguish between occasional 451 errors and consistent failures for specific senders or content types to identify a pattern. Consistent errors suggest deeper problems than temporary server issues.
• Examine email content: Review email content for any elements that might trigger filters, such as unusual encoding, overly large message sizes, or problematic attachments. This is crucial for troubleshooting deliverability, as discussed in our guide on why your emails fail.
• Monitor sender reputation: Actively manage and improve your sender reputation with Netease. Focus on maintaining low complaint rates, high engagement, and consistent sending volumes. More insights into managing IP reputation can be found in our discussion on temporary rate limiting due to IP reputation.
• Understand 451 specifics: While generic, a 451 error signals a temporary problem on the recipient server's end. This often means the server is overwhelmed or experiencing an internal issue, as detailed in this article on SMTP error 451.

Key opinions

• China's unique landscape: Many marketers acknowledge that Chinese ISPs, like Netease, operate differently from Western counterparts, often requiring a specialized approach to deliverability.
• Reputation building: It's commonly believed that building a sending reputation with Netease involves not just good engagement metrics but also demonstrating consistent and significant sending volumes, even if it means initially encountering a higher bounce rate.
• Error code interpretation: While 451 is generally temporary, marketers note that Netease may use generic error codes, making it difficult to pinpoint the exact cause of rejection if it's not a standard reputation issue.
• Partnership preference: A frequent suggestion is to work with in-country partners or ESPs that specialize in Chinese email delivery to navigate the complex local regulations and infrastructure challenges.

Key considerations

• Localized strategy: Adopt a strategy specifically tailored for the Chinese market, which might include ensuring content relevance, appropriate language, and adherence to local legal requirements.
• Content compliance: Pay close attention to all aspects of email content, including encoding and attachments, as these are common triggers for temporary rejections. This aligns with general advice for fixing emails going to spam.
• Volume scaling: Be prepared to scale your sending volume to Netease gradually, accepting initial bounces as part of the reputation-building process, rather than immediately backing off.
• Server overload checks: While it could be content, a 451 (4.3.0) can also be a simple server overload, which may resolve itself with retries. This is a common aspect of SMTP error messages.

Key opinions

• Deep technical analysis: Experts look beyond the generic 451 code to determine the exact stage of the SMTP conversation where the error occurs, as this provides crucial context (e.g., during HELO, MAIL FROM, RCPT TO, or DATA).
• Behavioral filtering: Chinese ISPs, particularly Netease, heavily rely on behavioral filtering, where a sender's historical performance, engagement rates, and even the sending volume influence temporary acceptance or rejection, even if authentication is perfectly aligned, as discussed in a simple guide to DMARC, SPF, and DKIM.
• Infrastructure considerations: The Great Firewall and localized internet infrastructure can significantly impact cross-border email delivery, leading to connection timeouts and 451 errors, regardless of sender reputation.
• Policy enforcement: Netease and other Chinese ISPs are subject to strict local regulations, which can lead to dynamic and often opaque filtering decisions that result in temporary rejections like 451s.

Key considerations

• SMTP transaction logging: Thoroughly review SMTP logs to pinpoint the exact moment the 451 error is returned. This can reveal if the issue is with initial connection, sender/recipient validation, or during content transfer. More information on such errors can be found in our article on common causes of SMTP 550 errors.
• Proactive monitoring: Implement continuous blocklist monitoring and perform regular deliverability tests specifically targeting Netease to detect reputation shifts or technical issues early.
• Content adaptation: Beyond technical aspects, consider the cultural and regulatory sensitivities of your email content for the Chinese market. Even seemingly innocuous content can trigger filters. Word to the Wise offers insights into email deliverability issues more broadly.

Key findings

• Standard definition: According to RFCs, a 451 error is a 'requested action aborted: local error in processing.' It's a temporary failure, prompting the sending server to retry later.
• Sub-code meaning: The 4.3.2 sub-code typically denotes a 'mail system internal error', suggesting an issue within the recipient's mail server itself, preventing it from processing the message at that moment.
• RFC 5321 context: This error category is part of SMTP's mechanism for signaling transient problems, distinguishing them from permanent rejections like 5xx errors, ensuring proper handling of email delivery attempts.

Key considerations

• Interpretation variations: While RFCs provide a foundation, individual mail servers can interpret and apply these codes with nuances, especially in highly regulated or unique network environments. The discrepancy between standard and practical interpretation is also seen in discussions on what RFC 5322 says vs. what actually works.
• Retry mechanisms: The 451 code explicitly signals that the sending server should retry delivery later. Proper retry logic is crucial for handling these temporary deferrals effectively.
• Beyond RFCs: For specific destinations like Netease, the 451 4.3.2 error may not solely reflect a transient internal server issue but could be a generic response to content filtering, reputation issues, or policy enforcement that isn't explicitly defined in standard RFCs. This highlights why understanding DMARC tags, as outlined in our list of DMARC tags, is also important for comprehensive deliverability management.