How to troubleshoot Cox.net 421 AUP#CXCNCT email rejections?
Matthew Whittaker
Co-founder & CTO, Suped
Published 20 Apr 2025
Updated 19 Aug 2025
8 min read
Dealing with email rejections can be a frustrating experience, especially when the bounce messages seem to contradict your server configurations. I've observed a recurring issue with Cox.net email rejections that present a specific challenge: the 421 AUP#CXCNCT error. This code typically indicates that the sending server has exceeded the allowed concurrent connection limit, usually set at 10. However, in many cases, including my own observations across numerous senders, the MTAs are configured for a maximum of one connection to Cox.net per sending IP.
The perplexing part is the pattern of rejections. It's not a consistent block, but rather an intermittent throttling. What I've seen is that Cox.net will accept approximately 5-10 emails from a given IP, then reject all subsequent attempts for about an hour, only to resume accepting a small batch before repeating the rejection cycle. While there are occasional exceptions where up to 100 emails might be accepted, these are rare.
This peculiar behavior, affecting a wide variety of IP addresses and hundreds of senders, points away from a simple sender misconfiguration. It strongly suggests that Cox might be misapplying the AUP#CXCNCT error code. This pattern, which began around November 5th and has been escalating, indicates a broader issue within Cox's email filtering or reputation management systems.
Deciphering the 421 AUP#CXCNCT error
The 421 SMTP error is a transient (temporary) failure, meaning the sending server should retry delivery. According to the SMTP error code definition, a 421 response typically means the service is unavailable, often due to too many connections. When you see AUP#CXCNCT, it's Cox's specific addition, indicating an Acceptable Use Policy violation related to concurrent connections.
However, the observed behavior of accepting a few emails then rejecting for an hour, regardless of low connection settings, suggests that Cox's system might be using this code for something other than strict concurrent connection limits. This could be a form of dynamic throttling or a response to perceived abuse, which is being incorrectly flagged as a connection issue. It might be a reputation problem masked as a technical one.
Understanding this discrepancy is crucial for troubleshooting. If your MTA is indeed configured for minimal concurrent connections, then the error message isn't giving you the full picture. The actual issue could be related to content filtering, sender reputation, or even a system-wide glitch on Cox's end. This requires looking beyond the literal meaning of the error code to diagnose the problem effectively.
Investigating potential causes
When facing these ambiguous rejections, the first step is to confirm your own server's behavior. Ensure your Mail Transfer Agent (MTA) is strictly adhering to a low concurrent connection limit to Cox.net domains. If you're running Postfix, you can configure this in your main.cf file to prioritize specific delivery to Cox.net.
Then, in your /etc/postfix/transport file, ensure you have an entry like:
Postfix transport configuration for Cox.nettext
cox.net cox_transport:
After making these changes, remember to run postmap /etc/postfix/transport and reload Postfix. Even if you've done this, double-checking confirms that the issue isn't on your end regarding connection limits. If you're still seeing these errors despite these settings, it reinforces the idea that Cox's system is misreporting the actual cause.
This leads us to consider external factors, such as Cloudmark. Cox uses Cloudmark for some of its spam and blocklist filtering. A potential listing on Cloudmark could cause your emails to be flagged, leading to the erratic throttling behavior, even if the error message suggests something else. Regularly checking your IP status on various blocklists (or blacklists) is a good practice to proactively identify potential issues. I've found that sometimes, these transient errors are a soft warning before a hard blocklist event.
Strategies for troubleshooting and mitigation
Since the issue appears to stem from Cox's end misinterpreting the actual cause of the rejection, direct communication is often the most effective route. I recommend contacting the Cox postmaster team or abuse desk, specifically at unblock.request@cox.net. Provide them with detailed logs, including timestamps, sending IPs, recipient addresses (if sensitive, use examples or anonymize), and the exact bounce messages with the AUP#CXCNCT code. Clearly state that your concurrent connection settings are well below their stated limit.
While awaiting a response, it's prudent to implement or refine your email throttling strategies. Rather than bombarding Cox.net with retries, implement a more conservative retry schedule that respects the observed hourly rejection pattern. This means waiting at least an hour before retrying messages to Cox.net recipients if a 421 AUP#CXCNCT bounce is received. This can help reduce the load on their servers and potentially alleviate the misclassification.
I often use a structured approach to prevent delivery issues with throttling, which means I try to manage email throttling for Cox.net. It's also vital to monitor your delivery rates and bounce patterns to understand why Cox.net emails soft bounce. If the delivered-to-rejected ratio continues to decline, it's a strong indicator that the underlying issue is reputational, not just technical. Keep a close eye on any other bounce codes you might receive, as they could provide further clues. You can find information about Cox email error codes on their support page.
Building and maintaining reputation
The long-term solution to avoiding issues like 421 AUP#CXCNCT and other similar rejections from ISPs such as Cox is to prioritize and consistently maintain excellent sender reputation. This isn't just about avoiding blocklists (or blacklists), but also about building a positive relationship with receiving mail servers. A strong reputation can help you navigate ambiguous error codes and receive better treatment from filters.
Crucial to reputation management are robust email authentication protocols: SPF, DKIM, and DMARC. These protocols verify that your emails are legitimate and prevent spoofing. Properly configured DMARC in particular can provide valuable insights into your email stream through reporting, allowing you to quickly identify and address potential authentication failures or abuse. I also strongly recommend understanding why your emails might be going to spam for a holistic view.
Beyond technical configurations, list hygiene plays a significant role. Regularly cleaning your email lists of inactive or invalid addresses reduces bounces and spam trap hits, both of which negatively impact your sender reputation. Finally, consistently sending relevant and engaging content to an opted-in audience fosters positive engagement, which email providers like Cox.net highly value.
Views from the trenches
Best practices
Proactively monitor your sending IP's reputation across various blocklists and internal reputation services.
Implement granular throttling policies, especially for ISPs like Cox that exhibit specific rate-limiting behaviors.
Maintain meticulous list hygiene to minimize bounces, spam complaints, and spam trap hits.
Common pitfalls
Misinterpreting a seemingly technical error code as the sole problem, when it may indicate deeper reputational issues.
Aggressively retrying emails to domains like Cox.net without adjusting for observed throttling patterns, which can worsen reputation.
Neglecting to contact the ISP's postmaster team directly with detailed logs for obscure error codes.
Expert tips
Engage directly with the ISP's postmaster or abuse desk, providing full context and logs for troubleshooting.
Recognize that some ISPs use generic error codes to mask underlying reputation or content filtering problems.
Leverage DMARC reports to gain deeper insights into email delivery and authentication failures, helping to pinpoint issues.
Expert view
Expert from Email Geeks says to be careful, as a similar Cox.net error code might indicate a block.
November 21, 2019 - Email Geeks
Expert view
Expert from Email Geeks says checking Cloudmark is crucial, as Cox uses them for filtering.
November 22, 2019 - Email Geeks
Navigating complex email delivery challenges
Troubleshooting Cox.net 421 AUP#CXCNCT email rejections requires a nuanced approach, especially when the error code appears to be a misdirection. While the code suggests concurrent connection issues, the observed hourly rejection patterns point towards a deeper, potentially reputation-based problem within Cox's filtering systems.
The key is to verify your own MTA settings, implement adaptive throttling, and proactively engage with the Cox postmaster team, providing them with detailed evidence. Simultaneously, bolstering your sender reputation through proper authentication (SPF, DKIM, DMARC) and maintaining pristine list hygiene will serve as your best long-term defense against these kinds of deliverability challenges.
Cox.net per sending IP.
Cloudmark. Cox uses Cloudmark for some of its spam and blocklist filtering. A potential listing on Cloudmark could cause your emails to be flagged, leading to the erratic throttling behavior, even if the error message suggests something else. Regularly checking your IP status on various blocklists (or blacklists) is a good practice to proactively identify potential issues. I've found that sometimes, these transient errors are a soft warning before a hard blocklist event.
