What causes 451 4.3.2 errors when sending email to Netease in China?
Matthew Whittaker
Co-founder & CTO, Suped
Published 20 May 2025
Updated 15 Aug 2025
8 min read
Encountering a 451 4.3.2 error when sending emails to Netease in China can be incredibly frustrating. While a 451 error typically indicates a temporary issue on the receiving server, this particular response from Netease often points to more persistent and specific challenges unique to the Chinese email ecosystem. It's not just a momentary glitch, but rather a signal that something deeper is at play, preventing your emails from reaching their intended recipients consistently.
The SMTP error code 451 generally suggests a temporary failure, often meaning the server is busy, experiencing resource issues, or employing measures like greylisting. In these cases, the sending server is expected to retry delivery later. However, when this error persists with Netease, it signals that standard retry mechanisms may not be sufficient to overcome the underlying issues.
Unlike typical temporary rejections, the 451 4.3.2 error from Netease (which includes domains like 163.com, 126.com, and yeah.net) can be symptomatic of deeper problems related to sender reputation, content, or even specific technical configurations. Understanding these nuances is crucial for improving your email deliverability to this challenging, yet significant, market.
Understanding the 451 4.3.2 error
The 451 4.3.2 error from Netease often goes beyond a simple temporary server overload. While the generic 451 code suggests a transient issue, the 4.3.2 Internal server error specific to Netease can point to a variety of factors unique to their mail handling. It's not always about exceeding server limits, but sometimes about how your email is perceived or processed by their systems. For a general overview of SMTP 451 errors, you can consult resources such as the CloudMailin SMTP troubleshooter.
In many cases, a 451 error is part of anti-spam measures, such as greylisting, which temporarily defers mail from unknown senders to see if legitimate servers will retry. While this is a common practice, Netease's implementation can be particularly stringent, sometimes leading to prolonged deferrals that resemble permanent blocks. Understanding how email greylisting works can provide some context, but Netease adds layers of complexity.
When you receive this specific error, it indicates that Netease's server is currently unable to process your request, often due to perceived issues with the sending IP, domain reputation, or message content. Unlike a 5xx hard bounce, a 451 means the receiving server is asking you to "try again later." However, if you see this error consistently for a particular sender or domain, it suggests that retrying alone may not resolve the problem without addressing underlying deliverability factors. For similar temporary rejection messages, see our article on Gmail's 451 4.3.0 mail server temporary rejection.
It's important to differentiate between a temporary error due to a busy server and a persistent 451 4.3.2 that acts as a soft block. If your emails are consistently failing to deliver with this specific error, you may need to look beyond simple retries and investigate your email practices. Many experts agree that persistent 451s often suggest an underlying issue that needs to be resolved rather than just waiting it out.
Netease and the unique challenges of China
Netease, a dominant email provider in China, operates under a unique set of rules and challenges compared to Western ISPs. Their systems are highly sensitive to sender behavior, and gaining a good reputation can be a complex, often counter-intuitive process. Unlike some other providers, Netease may initially assign a low reputation to new or infrequent senders, requiring a strategic approach to build trust.
A common characteristic of Netease's system is its expectation for senders to demonstrate consistent, high-volume sending with good engagement metrics. They may require senders to "hit" a certain volume, even if it initially results in bounces, to prove legitimate sending patterns. This means that if you send 5,000 emails per day and encounter bounces, Netease might expect you to increase your volume to 10,000, tolerating up to 5,000 bounces as part of their reputation-building process. This approach is very different from typical deliverability best practices.
Historically, there have been discussions around registration processes for domains and sending IPs with Chinese ISPs. While requirements can change, some email service providers (ESPs) might need to pay to register sending domains or work with local partners to ensure deliverability. This additional layer of complexity underscores the need for localized strategies for email deliverability in China. Working with a local partner can help navigate these specific regulations and requirements, including understanding Chinese email deliverability issues more broadly.
The persistent 451 4.3.2 error might also indicate Netease's systems are flagging something within your message content itself. This could range from specific keywords, unusual attachments, or even content encoding issues that their servers struggle to process. Given the unique internet infrastructure and regulatory environment in China, optimizing your content for local standards is paramount. For a comprehensive understanding of why emails fail, consider the specific challenges of the Chinese market.
Typical SMTP 451 behavior
General 451 errors are usually temporary, meaning the recipient server is temporarily unavailable or busy. The sending server is expected to retry automatically after a short delay.
Common causes include server overload, greylisting, or transient network issues. These are typically self-correcting or require minimal intervention.
Netease 451 4.3.2 nuances
With Netease, the 451 4.3.2 error, especially if persistent, often indicates a deeper-rooted issue. This could be related to poor sender reputation within China, message content, or lack of proper local registration.
Netease (like Netease.com) might interpret certain sending patterns or content as suspicious, leading to repeated temporary rejections that effectively block delivery without a hard bounce.
Building reputation with Netease
Netease requires a specific approach to sender reputation. Unlike many global ISPs, they often expect consistent, high-volume sending to prove legitimacy, even if it means initially encountering higher bounce rates. This domain reputation strategy is critical for establishing trust and overcoming persistent 451 4.3.2 errors.
Troubleshooting common causes and improving deliverability
Resolving persistent 451 4.3.2 errors to Netease requires a multi-faceted approach. First, review your message content for anything that might trigger their filters. This includes content encoding, language, message size, and the types of attachments. Emails with unusually large sizes or uncommonly used attachment types are often scrutinized more heavily. Ensuring your content is culturally appropriate and avoids spam triggers can significantly improve your chances.
Consider the technical aspects of your email sending. While the 451 4.3.2 error is often a response to the DATA command during SMTP conversation (meaning the message body is being rejected), it could also be triggered earlier, during the FROM or RCPT commands, indicating an issue with the sender's address or recipient. Always ensure your email authentication records (SPF, DKIM, DMARC) are correctly configured and aligned.
For ongoing issues, especially with high volumes, working with a local relay partner in China is highly recommended. These partners have established relationships with Chinese ISPs, understand the ever-changing regulations, and can offer IP space and infrastructure optimized for the region. This can help circumvent the strict rate limits and filtering often imposed on international senders. While this error is specific to Netease, general deliverability technical solutions might also offer some insights.
Example Netease 451 4.3.2 error messageSMTP
451 4.3.2 Internal server error
Factor
Impact on Netease Delivery
Recommendation
Sender Reputation
Low initial trust, requires high volume sending with good engagement to build.
Maintain consistent volume, monitor metrics, be patient.
Content Filtering
Specific keywords, attachments, or encoding issues can trigger rejections.
Optimize content for Chinese context, avoid suspicious elements.
Technical Compliance
Strict enforcement of email authentication (SPF, DKIM, DMARC).
Ensure proper SPF, DKIM, DMARC alignment.
Local Presence
International IPs often face heavy rate limits or unique filtering challenges.
Consider local ESPs or relay partners for optimal deliverability.
Navigating Netease deliverability
While a 451 4.3.2 error from Netease appears to be a temporary internal server error, its persistence often signals deeper challenges rooted in Netease's unique deliverability policies and China's internet landscape. Effectively sending emails to Netease requires a nuanced approach that goes beyond standard SMTP error troubleshooting.
By understanding Netease's reputation building requirements, optimizing content for local standards, and potentially leveraging local partners, you can significantly improve your chances of reaching the inbox. This proactive and adaptable strategy is key to overcoming these specific soft bounce issues and ensuring your email campaigns succeed in the Chinese market. It's about adapting your strategy for email deliverability issues in complex environments.
Views from the trenches
Best practices
Ensure your email authentication (SPF, DKIM, DMARC) is flawless, as Netease has strict checks.
Gradually increase sending volume to Netease domains to build a positive sending history with them.
Monitor your engagement metrics closely, focusing on low complaint and high open rates.
Optimize email content for the Chinese audience, avoiding triggers that might be flagged by local filters.
Consider engaging a local email service provider or relay partner within China.
Proactively check for any blacklisting of your sending IPs, as Netease might use private blocklists.
Verify that your DNS records are correctly configured and stable, as errors can lead to temporary rejections.
Common pitfalls
Treating Netease 451 errors like typical temporary deferrals and relying solely on retries without addressing underlying issues.
Sending inconsistent volumes, which can make it harder for Netease to establish trust in your sending patterns.
Ignoring content encoding, language, or attachment types that might be problematic for Chinese systems.
Not having a strategy for building reputation specifically within the unique Chinese email ecosystem.
Assuming that good deliverability elsewhere guarantees good deliverability to Chinese ISPs.
Failing to adapt to Netease's specific expectation of 'hitting' bounce limits to prove volume intent.
Overlooking potential domain or IP registration requirements that might exist for sending into China.
Expert tips
The 451 4.3.2 error is often related to something falling over or timing out during delivery, indicating a server-side breakdown rather than an intentional block.
Review content encoding, language, message size, attachments, and attachment content types, especially if the rejection occurs at the end of the DATA command.
Netease requires a specific reputation-building approach where consistent high volumes, even if they initially generate bounces, are needed to progress to higher reputation tiers.
There might be a registration process for domain names and sending IPs with Chinese ISPs, which could involve payment for ESPs.
A persistent 451 4.3.2 error could be a combination of reputation issues and an internal bug on Netease's side, leading to generic failure messages.
Working with a local partner to relay mail through China can be more effective than dealing with heavy rate limits and unique filtering from outside the country.
Having offices in China helps in understanding rule changes and directly engaging with ISPs like Netease.
Expert view
Expert from Email Geeks says a 451 4.3.2 error typically signifies a "something fell over / timed out during delivery" issue, indicating a problem on the recipient server's side rather than an intentional block. If some senders consistently see this error, they should investigate commonalities in their sending practices that might be causing Netease to experience issues.
2021-01-21 - Email Geeks
Expert view
Expert from Email Geeks says if the rejection occurs at the end of the DATA command, factors such as content encoding, language, message size, and attachment types should be examined.
Netease.com) might interpret certain sending patterns or content as suspicious, leading to repeated temporary rejections that effectively block delivery without a hard bounce.
