What causes 451 4.3.2 errors when sending email to Netease in China?
Matthew Whittaker
Co-founder & CTO, Suped
Published 21 Jul 2025
Updated 10 Aug 2025
9 min read
Summary
The 451 4.3.2 internal server error, often encountered when sending emails to Netease (163.com, 126.com, yeah.net) in China, is typically a temporary deferral message. Unlike a hard bounce, it suggests a transient issue, such as a server overload or a timeout during the email delivery process. However, for certain senders, this error can become persistent, indicating deeper underlying problems rather than just occasional network glitches.
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.
What email marketers say
Email marketers often face unique challenges when trying to reach inboxes in China, with Netease being a particularly complex destination. The consensus among marketers is that conventional deliverability strategies sometimes fall short due to the highly regulated and distinct nature of Chinese ISPs. Many have experienced similar trouble with Chinese domains, learning that successful delivery often requires adapting to local rules and infrastructure rather than just traditional SMTP best practices.
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.
Marketer view
Marketer from Email Geeks suggests that a 451 4.3.2 error might indicate something internal on the recipient server fell over or timed out during delivery, rather than an intentional block. This can happen occasionally, but if a sender sees it for every message, it points to a systemic issue related to their sending.
22 Jan 2021 - Email Geeks
Marketer view
Marketer from Email Geeks notes that if a 451 error occurs at the end of the DATA command, it's crucial to examine factors like content encoding, language used, message size, and the types of attachments included, as these can trigger server-side issues.
22 Jan 2021 - Email Geeks
What the experts say
Email deliverability experts highlight that while a 451 4.3.2 error is an SMTP temporary failure, its consistent occurrence, especially with Chinese email providers like Netease, points to more than just transient server issues. Experts emphasize the interplay of technical configurations, sender reputation, and the unique geopolitical and infrastructure realities of sending email into China. They often delve deeper into the SMTP conversation to understand where the error occurs, whether it's related to authentication, content, or rate limits.
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.
Expert view
Expert from Email Geeks observes that Chinese mail providers like Netease have unique and often opaque filtering mechanisms that extend beyond standard global practices. A 451 error, while temporary, could be a soft block based on unknown internal criteria, not just a server overload.
22 Jan 2021 - Email Geeks
Expert view
Expert from Email Geeks explains that the 451 4.3.2 error for Netease often signifies a timeout during the DATA phase. This suggests the content itself, or a specific aspect of it, might be causing the Netease server to process the message slowly or encounter an internal issue, leading to the deferral.
22 Jan 2021 - Email Geeks
What the documentation says
The 451 SMTP error code, as defined in RFCs, signifies a temporary failure. Specifically, RFC 5321 (which updates RFC 2821) details the various SMTP reply codes. A 4xx series reply (e.g., 451) indicates a transient negative completion reply, meaning the command was not accepted, and the requested action did not occur, but the condition is temporary and may be rectified by a reattempt. The 4.3.2 part is a sub-code, often indicating a mail system internal error. While the documentation provides a standard interpretation, real-world implementations by ISPs, especially in unique operating environments like China, can add layers of complexity that aren't explicitly covered by general RFCs.
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.
Technical article
Documentation from RFC 5321 states that a 451 reply code signifies a temporary negative completion reply. This means the command was not accepted, and the requested action did not occur, but the sending SMTP client should retry after some time.
01 Jan 2008 - RFC 5321
Technical article
Documentation from SMTP Error Codes Reference specifies that the 4.3.2 status code is categorized as a mail system internal error. This often points to issues like disk full, memory allocation failures, or other transient resource limitations on the recipient's server.
