What is the difference between concurrent connection limits and email throttling?

Concurrent connection limits restrict the number of simultaneous active connections your mail server can establish with Charter/Spectrum's receiving servers. This differs from throttling, which usually limits the total volume of emails sent over a period. Concurrent limits are about instantaneous connections, while throttling is about sustained sending rate.

Why do Charter/Spectrum and other ISPs impose concurrent connection limits?

Charter/Spectrum implements these limits to protect their infrastructure from being overwhelmed, prevent abuse, and manage traffic efficiently. They use it as a real-time defense mechanism against suspicious sending behaviors, which can include spam or malicious activity.

Does my sender reputation affect Charter/Spectrum's concurrent connection limits?

Yes, your sender reputation plays a significant role. IPs with good reputations might be allowed more concurrent connections, while newer or less trusted IPs (or those with a history of issues) will face stricter limits. Maintaining a strong reputation through good sending practices is key.

How can I identify if concurrent connections are causing my email bounces?

You should review your mail server's (MTA) or Email Service Provider's (ESP) bounce logs for specific error messages like "too many concurrent connections" or codes like AUP#1310. This indicates that your server is attempting too many simultaneous connections.

What is the primary solution for resolving concurrent connection bounce issues?

The primary solution is to reduce the number of simultaneous connections your mail server attempts to open to Charter/Spectrum. This is typically a configuration adjustment made by your ESP. You should provide them with the specific bounce message and request this change.

How to troubleshoot email bounce issues with Charter/Spectrum due to concurrent connections? - Troubleshooting - Email deliverability - Knowledge base

Dealing with email bounce issues to Charter/Spectrum recipients can be frustrating, especially when the bounce message points to too many concurrent connections. I've seen many senders encounter this, and it often leads to confusion because the immediate thought is throttling, but it's a bit more nuanced. This issue, sometimes manifesting with bounce codes like AUP#1310, indicates that your mail server (MTA) is attempting to open too many simultaneous connections to Charter's receiving servers.

It's a common misconception that this is purely about sending volume over time. Instead, it's about the number of parallel connections your server establishes at any given instant. Mailbox providers like Charter/Spectrum implement these limits to protect their infrastructure from overload and to detect suspicious sending patterns, such as those associated with spam or malicious activity.

Understanding concurrent connection limits

When you encounter a bounce message related to concurrent connections, it's a signal that your outgoing mail server is trying to initiate more parallel sessions than Charter/Spectrum's servers are willing to accept from your IP address at that moment. This is distinct from standard throttling, which typically limits the total number of messages or the volume of data sent over a period, regardless of the number of open connections.

These limits are often dynamic and can vary based on the sender's IP reputation. A strong, positive sending reputation might allow for more concurrent connections, while a newer or less trusted IP could face stricter limits. It's their way of managing traffic and preventing abuse, and it affects how quickly your emails can be delivered, even if your overall sending volume is low.

Concurrent connection limits

Mailbox providers use concurrent connection limits to protect their systems. They prevent a single sender from monopolizing server resources or overwhelming their infrastructure with too many simultaneous connections. This helps maintain service stability for all users and acts as a first line of defense against spam and denial-of-service attacks.

Identifying the bounce reason

The first step in troubleshooting any bounce issue is to precisely identify the error message. Charter/Spectrum's bounce messages are usually quite specific. Look for phrases like "too many concurrent connections" or specific bounce codes that refer to connection limits, such as AUP#1310. This code specifically means that the receiving mail server has rejected the connection because the sending IP has exceeded the allowed number of simultaneous SMTP connections. It is a common indicator of this type of blocklist (or blacklist) issue.

Reviewing your mail server's logs or your Email Service Provider's (ESP) bounce logs is crucial. These logs provide the exact bounce messages, including error codes and any accompanying text. This detailed information will confirm if the issue is indeed related to concurrent connections, rather than other common bounce reasons like content filtering or recipient issues.

Example Charter/Spectrum bounce messagetext

554 5.7.1 AUP#I-1310 Too many concurrent connections from your IP. Reduce your number of connections and try again later.

Even if you have contacted Spectrum or a filtering service like Cloudmark and they report no issues, the concurrent connection limits are often an internal policy. They are not always reflected as a traditional IP blocklist (or blacklist) entry but rather as a real-time rate-limiting measure. Understanding the precise bounce message allows you to communicate effectively with your ESP about the specific problem.

Strategies for resolution

The primary solution for concurrent connection bounce issues is to reduce the number of simultaneous connections your mail server attempts to open. This is a configuration change that typically needs to be made by your ESP or the administrator of your mail server. If you're using a dedicated IP address, you likely have more control or leverage to request these adjustments.

I often advise senders to approach their ESP with the specific bounce message and highlight that it's a concurrent connection issue, not just general throttling. While many ESPs have default traffic shaping rules, some mailbox providers may require more stringent, specific adjustments for certain IP addresses, especially if there's a perceived issue with sender reputation. This often requires a direct conversation about your sending patterns to Spectrum. If you're also experiencing broader deliverability issues, learning how to resolve email delivery issues with them is key.

Traditional throttling

Limits the rate of emails sent per unit of time (e.g., 100 emails per minute). This is about the volume over a period.

Focus: Total emails sent within a timeframe.
Impact: Slows down overall sending campaigns.

Concurrent connection limits

Limits the number of simultaneous SMTP connections an MTA can have open to a recipient server at any given instant. This is about parallel processing.

Focus: Number of open communication channels at once.
Impact: Can cause immediate bounces if too many connections are attempted.

Work with your ESP to implement or adjust connection policies specifically for Charter/Spectrum (and potentially other major ISPs). This might involve lowering the number of concurrent connections allowed to Spectrum's mail exchangers (MX records) or implementing a more sophisticated queue management system that respects these limits dynamically. If you are experiencing email throttling as well, addressing both might be necessary.

Advanced troubleshooting and best practices

Beyond adjusting concurrent connections, maintaining a healthy sending reputation is paramount. Your reputation directly influences the limits Charter/Spectrum imposes. This means continuously monitoring your email list hygiene, ensuring engagement, and avoiding spam traps. Regularly check your IP against common email blocklists (or blacklists) to ensure your sending infrastructure isn't flagged.

Implement and monitor email authentication protocols, including SPF, DKIM, and DMARC. These protocols help mailbox providers verify the legitimacy of your emails, which contributes significantly to your sender reputation. A well-configured DMARC record, for example, signals to receivers that you are serious about email security and deliverability. You can learn more about DMARC monitoring on our site.

Ensuring optimal deliverability

Monitor your bounces: Keep a close eye on your bounce rates, especially to specific domains like charter.net. Timely identification is key to resolving issues quickly.
Segment your sending: If possible, segment your email sends and adjust connection limits based on the recipient domain. Larger domains or those with stricter policies may require more conservative settings.
Communicate with your ESP: Provide your ESP with specific bounce messages and request adjustments to your concurrent connection limits for affected domains.

Finally, consider the overall health of your email program. High bounce rates, even if for concurrent connections, can negatively impact your sender reputation over time, leading to more aggressive filtering or placement on blocklists. Proactive management of your email sending infrastructure and list quality will prevent many such issues from arising.

Views from the trenches

Best practices

Actively monitor all bounce messages, not just for delivery failures but for specific reasons like concurrent connection limits.

Work closely with your Email Service Provider to configure connection limits (sessions per IP per minute) for specific domains, especially large ISPs like Charter/Spectrum.

Maintain a high sender reputation by sending to engaged recipients and regularly cleaning your email lists.

Ensure proper email authentication records (SPF, DKIM, DMARC) are in place and correctly configured.

Common pitfalls

Assuming a "too many concurrent connections" bounce is simply a general throttling issue without understanding the difference.

Not monitoring specific bounce codes and relying only on generic delivery failure reports.

Failing to communicate detailed bounce messages to your ESP, which can hinder their ability to help.

Overlooking sender reputation as a contributing factor to stricter concurrent connection limits imposed by ISPs.

Expert tips

Implement granular connection settings per recipient domain to optimize delivery while respecting ISP limits.

Use a dedicated IP address if sending high volumes to have more control over your sending behavior and reputation.

Regularly check your sending IP for any signs of being listed on a public blacklist (or blocklist), as this can worsen connection issues.

Engage directly with the postmaster of the affected ISP if standard adjustments don't resolve the problem, providing detailed logs and context.

Marketer view

A marketer from Email Geeks says that Spectrum limits the number of concurrent connections from a sender, as well as the total number of connections allowed, and these limits vary based on the reputation of the IP address. This client already throttles sends, but bounces are only happening about a third of the time, suggesting they might need to throttle more aggressively for concurrent connections.

2022-08-03 - Email Geeks

Expert view

An expert from Email Geeks says that concurrent connection issues are not traditional throttling; rather, the Mail Transfer Agent (MTA) sending the mail is opening too many connections at once.

2022-08-03 - Email Geeks

Navigating concurrent connection challenges

Troubleshooting email bounce issues related to concurrent connections with Charter/Spectrum requires a clear understanding of the difference between connection limits and general throttling. It's often not about the volume of email you send, but how many simultaneous SMTP connections your server is attempting.

By accurately identifying the bounce reason, adjusting your concurrent connection settings through your ESP, and proactively managing your sender reputation, you can significantly improve your email deliverability to Charter/Spectrum recipients. Consistent monitoring and adherence to best practices will help you navigate these technical challenges and ensure your messages reach the inbox.