Summary
Comcast email throttling can significantly impact open rates, leading to deferrals and bounces. A common issue is a 421 4.1.0 throttling error, particularly for new subdomains or IPs lacking sufficient sending history. This type of blocklist or blacklist action is often tied to Comcast's internal reputation systems, which assess both the volume and quality of mail from a given sender.
Key findings
- Throttling codes: The error message 421 4.1.0 156.70.24.118 Throttled - try again later indicates that Comcast is temporarily rejecting mail due to rate limiting. More details are available in the Comcast SMTP error codes.
- New subdomain impact: A new subdomain or IP (e.g., 30 days old) may be throttled due to a lack of sending history and established reputation.
- Volume and reputation: Comcast uses an internal reputation system based on historical quantity and quality of sends. Low volume on an IP will not help build the necessary history.
- Spam reports: Poor open rates are often directly correlated with emails being marked as spam by recipients. Gmail's spam rates are not indicative of what Comcast sees.
- Email type: Both transactional and marketing emails can experience throttling if sender reputation is low or volume is too high too quickly.
Key considerations
- Gradual ramp-up: Implement a slow and steady IP warming process for new IPs and subdomains to build trust with Comcast's systems.
- Spam rate reduction: Actively work to reduce spam complaints specific to Comcast recipients. Even with opted-in lists, user behavior can lead to spam marking. This is crucial for improving your Comcast email performance.
- Sending speed: Adjust your sending speed and volume to align with Comcast's internal rate limits. Over-sending, even legitimate mail, can trigger throttling. For more information, see how to prevent throttling.
- ISP-specific reputation: Understand that each ISP, like Comcast, maintains its own reputation systems, which may differ from other providers like Gmail. Monitoring these separately is key.
What email marketers say
Email marketers often face challenges with ISP throttling, particularly when dealing with newer IPs or varying list quality. Their focus typically revolves around understanding throttling triggers, managing sender reputation, and implementing strategies to ensure consistent inbox placement. Balancing sending volume with recipient engagement is a recurring theme among marketing professionals.
Key opinions
- Speed of sending: Many marketers agree that sending too fast, especially with a new IP or subdomain, is a primary cause of throttling by ISPs like Comcast.
- Engagement metrics: While marketers track overall engagement, it's often emphasized that ISP-specific metrics, such as spam reports at Comcast, are more critical than general metrics like Gmail spam rates when troubleshooting specific deliverability issues.
- List quality: Even with seemingly opted-in contacts, low engagement or high complaint rates can signal to ISPs that the mail is unwanted, leading to throttling or blocklisting.
- Dedicated IP vs. Shared: Marketers on dedicated IPs, while having more control, still need to build their own reputation, which includes managing sending volume and list hygiene.
Key considerations
- Segment audiences: Segmenting your email list, especially for different types of mail (transactional vs. marketing), can help in managing sending rates to specific ISPs like Comcast.
- Monitor specific metrics: Beyond general open rates, closely monitor bounce codes and throttling messages received from ISPs. For improving overall rates, consider strategies to improve deliverability.
- List hygiene: Regularly clean email lists to remove inactive or problematic addresses to reduce spam complaints and improve sender score. This is a key step to avoiding spam filters.
- Throttling solutions: Email throttling occurs when an ISP limits the number of emails sent within a specific timeframe. Marketers should focus on maintaining a good sender reputation and warming up new IPs to prevent it, as detailed by Campaign Refinery on email throttling.
Marketer view
A marketer from Email Geeks notes experiencing perfect inboxing everywhere except for Comcast, with deferrals and bounces showing a throttling code. They have been sending for about 30 days on a new subdomain, primarily transactional traffic, and are seeking Comcast-specific best practices.
Marketer view
An email user from Xfinity Community Forum complains that Comcast has overly strict throttling rules, stating that even a small number of inbound messages from a single server can lead to days of throttling, which they find absurd for their legitimate use.
What the experts say
Email deliverability experts highlight that throttling by ISPs like Comcast is a direct consequence of perceived sender reputation, which is influenced by historical sending patterns, volume, and quality (spam reports). They emphasize that ISP-specific data is far more relevant than general metrics like Gmail spam rates when troubleshooting throttling issues. Consistent, quality sending is crucial for building trust and avoiding blocklists.
Key opinions
- Sending speed: Experts commonly attribute throttling to sending too quickly, especially on newer sending IPs or subdomains.
- Internal reputation systems: Comcast, like other major ISPs, utilizes an internal reputation system that considers the sender's historical volume and quality of mail, directly impacting how much mail they will accept. This system is distinct from other ISPs.
- Relevance of data: A low spam rate at one ISP (e.g., Gmail) does not indicate good reputation or deliverability at another ISP (e.g., Comcast). Each ISP's metrics should be analyzed independently.
- Impact of spam reports: Poor open rates and throttling are often a direct result of mail being marked as spam by recipients, regardless of whether the list is opted-in. This negatively affects sender reputation.
- IP warming importance: For new IPs, a gradual increase in sending volume (IP warming) is crucial for building trust and establishing a positive sending history with ISPs.
Key considerations
- Analyze bounce codes: Pay close attention to specific rate limiting (RL) codes provided by ISPs, as they offer direct insight into why mail is being throttled. This will assist you in improving deliverability and open rates.
- Sender reputation: A strong sender reputation is vital for email deliverability. Factors such as bounce rates, sending history, and spam complaints significantly influence domain reputation, as explained by MailMonitor in its glossary.
- Consistency in sending: For IPs without established volume history, consistent, albeit slow, sending helps build the necessary reputation over time. This is part of a practical guide to domain reputation.
- Warm-up process: IP warming is a critical component for high email deliverability. Following a structured guide to build trust with ISPs, improve reach, and increase inbox placement is highly recommended, as highlighted by Knak on IP warming.
Expert view
An expert from Email Geeks suggests that 'throttled' generally means the sender is sending too fast. They advise slowing down a little, especially if the issue is related to a new subdomain.
Expert view
An expert from SpamResource explains that throttling issues often stem from new or recently changed sending IPs or domains that lack sufficient positive sending history. They advise a slow and steady ramp-up of volume.
What the documentation says
Official documentation and research often explain that ISPs employ sophisticated filtering mechanisms, including throttling, to protect their networks and users from unwanted mail. These mechanisms rely heavily on sender reputation, which is built over time through consistent, high-quality sending. Documentation typically advises senders to comply with best practices, manage sending volume, and monitor specific error codes to mitigate deliverability issues.
Key findings
- ISP role: ISPs play a crucial role in email deliverability, acting as gatekeepers that determine which emails reach the inbox. They can block or throttle emails based on various factors, including sender reputation.
- Sender score: A sender score or sender reputation is a critical factor for ISPs. It's influenced by metrics like bounce rates, spam complaints, engagement, and sending history.
- Throttling definition: Email throttling is a legitimate practice by ISPs to limit the number of emails accepted from a sender within a given timeframe, often triggered by a sudden increase in volume or signs of low reputation.
- Bandwidth vs. Email Throttling: While internet throttling can occur for high bandwidth usage (streaming), email throttling is distinct and specifically relates to mail server capacity and sender reputation metrics.
Key considerations
- Authentication standards: Proper implementation of email authentication protocols like SPF, DKIM, and DMARC is fundamental for building trust with ISPs and improving deliverability. For more information, read a simple guide to DMARC, SPF, and DKIM.
- User engagement: Ensuring email send frequency aligns with end-user engagement levels is a best practice. Using frequency capping can limit emails sent per user per day/week, as outlined by MoEngage's best practices.
- Postmaster tools: Leverage ISP-specific postmaster tools, such as Google Postmaster Tools, to monitor reputation, spam rates, and delivery errors, even if the data isn't directly transferable across all ISPs.
- Avoiding spam: Spam filters examine various factors, including content, sender reputation, and engagement, to determine inbox placement. Following best practices is key to maximizing delivery rates and avoiding the spam folder. For more on this, see why your emails go to spam.
Technical article
Documentation from Campaign Refinery defines email throttling as an ISP limiting the number of emails sent within a specified timeframe. They explain that understanding why it happens and how to prevent it is crucial for consistent email deliverability.
Technical article
An article from Knak highlights that IP warming is essential for high email deliverability. It suggests following a structured guide to build trust with ISPs, improve reach, and increase inbox placement, which is particularly relevant for new senders.
Related resources
8 resources
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