How to resolve email throttling issues with Charter.net when sending high volumes of email?

Key findings

• Persistent Throttling: Despite a strong overall sender reputation (green scores on Google Postmaster Tools and SNDS, no blocklist incidents), Charter.net can impose severe throttling limits, impacting deliverability for high-volume senders.
• Error Code Specificity: The common error code 550 5.1.0 sender rejected. AUP#In-1310 points to specific policy violations or reputation concerns as interpreted by Charter's systems. More information can be found in Charter's support documentation on understanding email error codes.
• ISP-Specific Reputation: Different Internet Service Providers (ISPs) maintain unique and varied reputation standards, meaning a good reputation with one ISP does not guarantee the same with another.
• Volume Sensitivity: Large volume spikes, even from warmed-up IPs, can trigger throttling from Charter, especially during high-traffic periods like GivingTuesday.
• Limited Communication: Direct communication channels, such as unblock@charter.net, are often unresponsive, making direct issue resolution challenging.

Key considerations

• Review Throttling Limits: Carefully assess Charter's actual message acceptance rate (e.g., 65 messages per hour) and ensure your sending infrastructure respects these limits to avoid further deferrals.
• Adjust Sending Patterns: If current sending strategies (e.g., 15 messages per minute, 3 connections per minute) result in throttling, it's necessary to decrease volume and concurrent connections to below the observed limits, even if it seems excessively slow. This may require updating your email throttling prevention settings.
• Investigate Deeper Reputation: Even with positive general reputation, Charter's specific internal filters might be flagging content or sender behavior. Consider analyzing message content, recipient engagement patterns, and any potential blocklist placements on less common local blocklists (using a blocklist checker is a good starting point).
• Prepare for Peak Volume: For anticipated high-volume events, plan weeks or months in advance to gradually increase volume to specific ISPs like Charter to build historical trust, ideally not exceeding 2x normal peak volume.
• Explore Alternative Contact Methods: Since direct email contact is often ineffective, seek out other communication channels or industry forums where ISP representatives might be present.

Key opinions

• ISP Discrepancy: Many marketers report that despite green reputation scores on tools like Google Postmaster Tools and SNDS, and high deliverability to other major providers, Charter.net (Spectrum) disproportionately throttles their email.
• False Positive Concerns: A common belief among senders is that Charter's throttling, particularly for high-volume non-profit mail, might be a false positive related to the sheer volume rather than actual spam indicators, especially if engagement is strong.
• Unresponsive Postmaster: Marketers frequently express frustration over the lack of response from Charter's unblock email addresses, making direct communication and resolution difficult.
• Volume Impact on Special Events: Throttling becomes particularly impactful during peak sending days like GivingTuesday, where volumes can multiply significantly, leading to delayed or undelivered messages to Charter recipients.

Key considerations

• Accepting ISP Standards: While frustrating, it's generally accepted that ISPs like Charter.net have their own unique, often opaque, reputation standards. Senders must ultimately conform to these, even if they differ from other major providers. This is a common issue with providers such as Yahoo throttling as well.
• Incremental Volume Increases: Even with seemingly warmed-up IPs, a sudden 5x increase in volume for a specific provider like Charter, even if prepared for, may still trigger their internal abuse prevention systems, leading to throttling. Consider slower, more gradual volume ramp-ups to that specific domain.
• SMTP Mail From Impact: Changing the SMTP MAIL FROM string can sometimes influence how an ISP's filters perceive mail. While not a permanent fix, it might offer temporary relief or diagnostic clues if the throttling is tied to specific sender identities, or if you are troubleshooting why your emails are going to spam.
• Resource Allocation: If Charter.net recipients constitute a small percentage of total volume (e.g., less than 1%), marketers may need to weigh the effort of resolving severe throttling against its overall impact on campaign goals, especially if it diverts resources from more impactful deliverability efforts for larger segments.

Key opinions

• Reputation is Subjective: Deliverability experts firmly state that different ISPs (Internet Service Providers) have varying reputation standards. If Charter.net is throttling, it signifies that, from their perspective, there is a reputation issue, regardless of other ISP's views.
• Concurrency vs. Rate Limits: Experts note that throttling might be due to a concurrency limit (how many simultaneous connections) rather than a simple rate limit (messages per minute). This distinction requires examining specific bounce messages related to concurrent connections or smarthost settings/logs for diagnosis.
• Slow and Steady Wins: A common expert recommendation is to slow sending down to below the observed throttling limit, even if it feels extremely slow, to rebuild trust and gradually increase sending speeds over time.
• Pre-Event Warm-up: For significant volume spikes during special events, experts advise months of preparation to build up sufficient volume for the sending IP and domains. It's unrealistic to expect ISPs to allow sudden, large increases (e.g., 5x normal volume) without prior consistent traffic at those levels.
• Indirect Contact: Given the unresponsiveness of direct contact channels like unblock@charter.net, experts suggest exploring forums like Mailop, where ISP representatives might monitor discussions even if they don't actively post.

Key considerations

• Underlying Problem Identification: If changing elements like the SMTP Mail From string results in similar throttling, it strongly suggests an underlying issue with how Charter's filters perceive the mail as spam. The focus should shift to identifying and fixing that root cause, which can be explored in a general guide to email deliverability issues.
• Proactive Volume Scaling: Instead of reacting to throttling during peak events, plan to gradually scale up volume to Charter recipients over an extended period. A rule of thumb suggests not exceeding 2x normal peak volume for any given send. This is also important when dealing with general throttling from providers like Spectrum/TWC.
• Question Blocking Decisions: If a sender has meticulously warmed up IPs and prepared for high volumes, experts advise questioning the blocking and requesting remediation from Charter, even if direct contact is challenging.
• Analyze Logs for Concurrency: Thoroughly review SMTP logs and smart host settings to differentiate between rate limits and concurrency limits. Understanding which limit is being hit is crucial for adjusting sending parameters correctly.

Key findings

• Dynamic Throttling: ISPs employ dynamic throttling mechanisms that adjust based on various factors, including sender reputation, network traffic, and real-time spam detection, to maintain network stability and protect users.
• Error Code Guidance: Error codes like AUP#In-1310 from Spectrum (Charter) provide specific, albeit sometimes general, reasons for rejection or throttling. These codes often indicate policy violations related to acceptable use or sender reputation. Their documentation, such as understanding email error codes, is the primary source of truth.
• Reputation is Key: ISP documentation consistently emphasizes that a positive sender reputation, built on low complaint rates, high engagement, and proper authentication, is the fundamental defense against throttling and blocking.
• Authentication Standards: Full compliance with email authentication protocols (SPF, DKIM, and DMARC) is a baseline requirement. Issues with these can lead to immediate filtering or throttling. Ensuring DMARC, SPF, and DKIM alignment is critical.

Key considerations

• Adaptive Sending: Documentation often advises senders to implement adaptive sending algorithms that automatically adjust volume and connection rates based on real-time feedback (bounces, deferrals, and throttling). This allows systems to back off gracefully and retry later.
• List Hygiene: Maintaining a clean and engaged mailing list is paramount. Sending to invalid or inactive addresses can generate bounces and spam traps, negatively impacting reputation and increasing the likelihood of throttling.
• Feedback Loops: Registering for and actively monitoring ISP feedback loops (FBLs) allows senders to track user complaints and remove complaining subscribers promptly, preventing further reputation damage that could lead to throttling or email delivery blocks.
• Content Quality: Even for legitimate mail, content that mimics spam or has poor formatting can trigger filters. Regular content audits and A/B testing can help identify problematic elements.