What should I do about Comcast throttling and deferral errors?

Key findings

• Temporary failures: The '421 4.1.0 Throttled' error and 'Too many sessions opened' are typically temporary, leading to deferrals rather than permanent rejections. Emails are usually delivered after subsequent retries.
• Rate limits: Comcast's throttling (like the RL000003 error code) is a volume-based rate limit. It indicates that you are sending too many messages, or too many concurrent connections, within a given timeframe.
• Reputation factor: While volume-based, Comcast's throttling can also be influenced by your historical sending quantity and quality. Poor sender reputation can lead to stricter rate limits. For more information on this, read about understanding your email domain reputation.
• Engagement confirmation: If deferred emails are eventually delivered and users engage with the content, it suggests that the core issue is related to sending volume or speed, rather than content or outright blocking.

Key considerations

• Sending slower: The primary solution for throttling is to reduce your sending speed and limit the number of concurrent connections to Comcast's mail servers. This can involve spreading out campaigns or adjusting your sending infrastructure's settings.
• Impact on user experience: While emails eventually deliver, delays can negatively impact time-sensitive communications (e.g., transactional emails) and user experience. Consistent deferrals may also lead to user complaints or reduced engagement.
• Monitoring: Even if delivered, tracking deferral rates is important. A sudden increase can indicate a change in your sending reputation or Comcast's policies. Regularly monitor your sender reputation and bounce logs.
• Proactive adjustments: Don't wait for issues to become severe. Proactively manage your sending volume and warm up new IPs, especially for transactional emails to domains like Comcast. Learn more about how to prevent email throttling and delays.

Key opinions

• Frustration with rate limits: Many marketers report being frustrated by Comcast's rate limiting, especially when sending large volumes of mail or to specific domains. This is a common pain point for many senders.
• Contacting support: Some marketers suggest contacting Comcast's Customer Security Assurance (CSA) directly for rate limiting issues, though results may vary. More often, it's about optimizing your own sending practices.
• Connection reset errors: Marketers also experience 'connection reset' errors, which might be another form of throttling or an indication of network overload at Comcast's end.
• Seeking bypass methods: Some users actively look for ways to bypass (or work around) Comcast throttling for general internet usage, which reflects the broader impact of throttling policies on user experience beyond email.

Key considerations

• Campaign timing: Spreading out email campaigns over longer periods, especially those targeting Comcast addresses, can help avoid 'Too many sessions opened' errors. This is crucial for maintaining good deliverability rates and avoiding issues like temporary deferral errors on other large ISPs.
• Deliverability confidence: Even with eventual delivery, the concern about initial deferrals highlights the need for continuous monitoring and optimization to ensure messages are received promptly.
• Domain reputation: Low or zero open rates to Comcast users might indicate deeper deliverability problems, potentially related to your domain's reputation. Addressing these can reduce throttling incidence. See how to troubleshoot Comcast 0% open rates.
• Beyond email throttling: It's important to differentiate email-specific throttling from broader internet throttling. While the latter might affect general network performance, email deliverability issues are usually tied to SMTP-level rate limits based on sender behavior. For insights into general internet throttling, see how to test for internet throttling.

Key opinions

• Adjust sending speed: Experts agree that senders are being instructed to reduce their sending speed and limit the number of connections to Comcast's mail infrastructure to mitigate throttling.
• Retry aggressively: While RL000003 (a common Comcast deferral code) is considered retryable, experts caution against aggressive retries, as it's not an invitation to overwhelm the system.
• Historical data matters: Comcast's throttling isn't solely based on current volume; it also considers historical sending quantity and quality (i.e., your sender reputation). This aligns with general ISP best practices regarding email deliverability issues.
• Deferral interpretation: A few minutes of deferral time is generally considered acceptable and manageable, distinguishing it from longer deferrals (hours) experienced with some other mail providers.

Key considerations

• Proactive outreach: An expert suggests sending specific IP addresses to Comcast for review, indicating a potential avenue for direct assistance in resolving persistent throttling issues.
• Understanding deferral nuances: While a deferral code like RL000003 might seem benign because it's temporary, ignoring the underlying cause (sending too fast or poor reputation) can lead to more severe issues over time. This is similar to how Yahoo throttles email messages.
• Mailbox provider expectations: The different retry expectations for various mailbox providers (MBPs) highlight the need for adaptable sending systems that can respond to varying deferral signals.
• Avoid aggressive tactics: Attempting to 'run over' or force emails through rate limits is strongly discouraged, as it can worsen sender reputation and lead to longer-term blocklisting or blacklisting by ISPs like Comcast.

Key findings

• Temporary deferral: Comcast documentation, such as the postmaster SMTP error codes, confirms that 421 errors (e.g., 'Throttled') are temporary failures, not permanent rejections.
• Rate limit specifics: Error codes like RL000003 specifically indicate a rate limit violation based on the volume of mail being sent to Comcast's infrastructure.
• Retry mechanism: The 421 response implies that the sending server should attempt to deliver the message again after a certain period, suggesting that the issue is not with the content or authenticity of the email itself.
• Beyond immediate volume: While documented as rate limits, implicit in ISP documentation is that consistent violations can lead to more severe reputation issues over time, moving from temporary deferrals to more stringent blocklists or blacklists.

Key considerations

• SMTP standards: The 4xx SMTP error codes are standard across email systems for temporary failures, requiring the sender to retry. This is fundamental to email deliverability and how mail servers handle transient issues.
• Sender responsibility: Documentation implicitly places responsibility on the sender to manage their sending rates and connection limits to comply with the receiving server's capacity and policies. This involves actively managing your sender reputation, which is key to avoiding issues like Comcast email throttling affecting open rates.
• Policy updates: Postmaster documentation is subject to change, reflecting evolving anti-spam and network management policies. Senders should periodically check for updates that might affect their delivery to Comcast.
• Configuration for deferrals: Server configurations (e.g., Postfix transport maps) can be adjusted to treat specific domains, like Comcast, differently to reduce deferrals by managing retry schedules. This is a common practice for SMTP 421 errors with Web.de and GMX.net.