What causes Comcast email throttling and how can I improve my open rates?
Matthew Whittaker
Co-founder & CTO, Suped
Published 29 May 2025
Updated 16 Aug 2025
10 min read
Email throttling is a common challenge for senders, especially when dealing with major Internet Service Providers (ISPs) like Comcast. It occurs when a receiving mail server intentionally limits the number of emails it will accept from a particular sender within a specific timeframe. You might see this manifest as a 421 4.1.0 Throttled - try again later error message, indicating that Comcast's servers are temporarily rejecting your mail because they perceive you're sending too fast or have concerns about your sending practices. This can severely impact your email deliverability and, consequently, your open rates.
The core reason for throttling is to protect their network and users from spam and abuse. While frustrating, it is a mechanism ISPs use to maintain email ecosystem health. Understanding the factors that trigger this behavior and implementing best practices is key to navigating Comcast’s systems effectively and improving your overall inbox placement.
Why Comcast throttles your email
Comcast, like other large ISPs, employs sophisticated filtering systems that analyze various signals to determine whether to accept an email, delay it, or reject it. One of the primary drivers of throttling is your sender reputation. This is an internal assessment by Comcast (and other ISPs) of the historical quality and quantity of mail originating from your IP addresses and domains. A newer IP or subdomain, without a long history of sending good mail, is often treated with more caution.
If you're sending from a new dedicated IP or subdomain, as was the case for a client recently, Comcast’s systems might not have enough historical data to trust your sending volume. They operate on a principle where they won't automatically grant a huge volume capacity to an IP that hasn't demonstrated a consistent need for it over time. This leads to rate limiting, where your messages are deferred with the intention that you slow down and try again later. It is a soft bounce, but it still negatively impacts your deliverability and email open rates. To understand more about these errors, you can refer to Comcast's email error codes.
Beyond volume, the quality of your mail also plays a significant role. If your emails are frequently marked as spam by recipients, or if you hit spam traps, your sender reputation will suffer, leading to stricter throttling or even outright blocklisting (blacklisting). Even with 100% opted-in lists, unexpected spam reports can occur, especially if content or sending frequency doesn't align with recipient expectations. Comcast, like other ISPs, allows a throttle rate based on your sender reputation and authentication. This means a poor reputation will trigger more aggressive throttling.
Understanding the 421 4.1.0 error
The error code 421 4.1.0 156.70.24.118 Throttled - try again later specifically points to a rate limiting issue. This isn't a permanent block, but a temporary deferral. Comcast is telling you that the IP (in this case, 156.70.24.118) is sending too quickly for its current reputation at Comcast. To learn more about common throttling issues, read our article on how to prevent email throttling.
Rate Limiting: The ISP is limiting the volume of email from your sending IP.
Temporary Error: Your email will typically be retried by your sending system.
Sender Reputation: This issue is often tied to a nascent or less-than-stellar reputation with Comcast.
The role of sender reputation and IP warm-up
Building a solid sender reputation with Comcast, and indeed any ISP, is a gradual process. When you start sending from a new IP or domain, you need to go through an IP warm-up phase. This involves gradually increasing your sending volume over time, allowing ISPs to observe your sending behavior and establish trust. Trying to send too much too soon, even to opted-in contacts, can trigger throttling because your IP hasn't built sufficient historical quantity or reputation.
Comcast's systems are designed to detect unusual sending patterns. If an IP that historically sends very little suddenly attempts a high volume, it raises a red flag. The system prioritizes IPs with a demonstrated history of sending legitimate, wanted mail. This is why a low volume of mail from a new IP can still lead to throttling if the system anticipates a larger volume than what it’s ready to accept. For example, if you send only a few dozen messages in two weeks from a new IP, Comcast's system won't suddenly give you capacity for thousands.
Open rates are a crucial indicator of engagement and play a direct role in your sender reputation. If your open rates are consistently low, especially for Comcast recipients, it signals to their systems that your emails might not be wanted, or worse, are perceived as spam. This can lead to increased filtering, including throttling and direct placement into the spam folder. It’s essential to monitor your open rates specifically for Comcast (and Xfinity) destinations, as overall metrics from other ISPs like Gmail may not reflect your standing with Comcast.
Bad practices
Rapid volume increase: Sending high volumes from a new IP or subdomain immediately.
Ignoring deferrals: Not adjusting sending speed despite 421 Throttled bounces.
High spam complaint rate: Receiving too many spam reports from Comcast users.
Good practices
Gradual IP warm-up: Slowly increase sending volume to establish trust and historical data.
Monitor and adapt: Adjust sending speeds based on real-time feedback and bounces.
Focus on engagement: Send relevant content to highly engaged subscribers to minimize complaints.
Essential authentication and content strategies
Beyond volume and recipient engagement, strong email authentication is non-negotiable for improving your open rates and avoiding throttling. This includes correctly configuring SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance). These protocols verify that your emails are legitimately coming from your domain, preventing spoofing and building trust with ISPs like Comcast. Without them, your mail is more likely to be flagged as suspicious, even if your content is excellent.
A key component of avoiding throttling is reducing spam complaints. Even with 100% opted-in lists, some recipients might mark your emails as spam if they no longer find the content relevant, they forgot they subscribed, or they simply prefer not to receive it. High spam complaint rates directly signal to Comcast that your mail is unwanted, severely damaging your sender reputation and leading to stricter throttling or blocklisting. It is crucial to monitor feedback loops and promptly remove users who complain.
Maintaining a clean and engaged email list is another critical strategy. Regularly remove inactive subscribers and unengaged contacts who haven't opened or clicked your emails in a long time. These stale addresses can lead to bounces, spam trap hits, and lower engagement metrics, all of which negatively impact your sender reputation and increase the likelihood of throttling. Sending only to those who actively engage with your content demonstrates to Comcast that your mail is valued.
Consider the relevance and value of your content. Even if a user opted-in, if the emails they receive don't meet their expectations or provide value, they are more likely to ignore them or mark them as spam. High engagement, characterized by opens, clicks, and replies, tells Comcast that your mail is wanted, which helps to mitigate throttling and improve your inbox placement. Review your content strategy to ensure it aligns with your subscribers' interests. For further assistance in improving deliverability, explore our article on why your emails fail.
Actionable steps to improve deliverability and open rates
The most immediate action you can take when experiencing Comcast throttling is to slow down your sending speed. If your ESP allows, adjust your sending rate to gradually increase it over time, mimicking a natural warm-up process. This patient approach allows Comcast's systems to build a positive history with your IP and domain. You might also consider segmenting your sends to Comcast recipients, sending them in smaller batches initially while closely monitoring your bounce and spam complaint rates.
Ensuring your email authentication records (SPF, DKIM, and DMARC) are correctly set up and aligned is fundamental. These records provide proof that your emails are legitimate, which builds trust with Comcast. Incorrectly configured or missing records can immediately trigger spam filters and throttling. Regularly check your DNS records to confirm they are accurate and accessible. For a comprehensive guide on setting up these records, refer to our article on DMARC, SPF, and DKIM.
Actively managing your email list is crucial for long-term deliverability. Implement a strict double opt-in process to ensure all subscribers genuinely want your emails. Regularly clean your list by removing inactive users, hard bounces, and any addresses that repeatedly mark your emails as spam. A smaller, highly engaged list will perform much better than a large, unengaged one, reducing your overall spam complaint rate and improving your sender reputation with Comcast. You can also monitor your email domain reputation regularly.
Finally, focus on the quality and relevance of your email content. Personalized, valuable, and engaging content is less likely to be marked as spam. Avoid spammy keywords, excessive images, or broken links. A high open rate and click-through rate signal positive engagement to Comcast, which reinforces your good sender reputation and reduces the likelihood of throttling. If your Comcast open rates are 0%, a comprehensive content and list hygiene review is essential.
Views from the trenches
Best practices
Implement a slow and steady IP warm-up process for new sending infrastructure, gradually increasing volume.
Maintain strong email authentication (SPF, DKIM, DMARC) to build trust with Comcast and other ISPs.
Actively monitor your bounce and spam complaint rates specifically for Comcast destinations.
Segment your audience and send highly relevant content to boost engagement and reduce complaints.
Regularly clean your email lists by removing inactive subscribers and hard bounces.
Common pitfalls
Sending high volumes too quickly from new IPs or subdomains, triggering immediate throttling.
Ignoring
421 Throttled
bounce messages and failing to adjust sending rates.
Failing to implement proper email authentication, leading to messages being flagged as suspicious.
Expert tips
Use separate IPs for transactional versus marketing emails if possible to isolate reputation.
Engage with Comcast's postmaster resources for specific guidelines and potential whitelist requests.
Ensure your content is personalized and provides clear value to prevent spam complaints, even from opted-in users.
Consider a dedicated IP if you have consistent, high-volume sending to maintain better control over your reputation.
A gradual increase in sending volume, even for transactional emails, helps build historical data and trust.
Expert view
Expert from Email Geeks says 'throttled' generally means you are sending too fast, and this might be related to a new subdomain, suggesting a slower sending rate is needed.
2022-06-20 - Email Geeks
Expert view
Expert from Email Geeks says that Comcast's throttling is based on an internal reputation system, which considers historical quantity and quality, so sending a low volume or being marked as spam will hinder deliverability.
2022-06-20 - Email Geeks
Our recommendations
Comcast email throttling is fundamentally a deliverability challenge rooted in sender reputation. To improve your open rates and ensure your emails reach the inbox, a multi-faceted approach is necessary. This involves carefully managing your sending speed, particularly when using new IPs or subdomains, and consistently demonstrating good sending behavior.
By prioritizing slow and steady IP warm-up, ensuring robust email authentication, maintaining a clean and engaged subscriber list, and delivering valuable content, you can build and sustain the positive sender reputation required to avoid throttling and achieve better inbox placement with Comcast and other ISPs. This ongoing commitment to best practices will ultimately translate into higher open rates and more successful email campaigns.
