What should I do about Comcast throttling and deferral errors?
Summary
What email marketers say9Marketer opinions
Marketer from Email Geeks shares that notes on RL000003 are that it can be retried pretty aggressively and not something impactful to deliverability since it's purely a volume based rate limit.
Email marketer from Reddit shares that Comcast Business Class sometimes has aggressive filtering and that contacting Comcast support directly to whitelist your sending IP can help resolve delivery issues.
Email marketer from EmailOnAcid explains that deferrals are a sign that something is wrong, and it is critical to examine your email program to identify potential issues, such as a poor sender reputation or exceeding sending limits. They advise using tools to monitor your deliverability.
Email marketer from Mailjet Blog suggests monitoring your sending reputation, warming up new IPs gradually, and adhering to best practices for email authentication (SPF, DKIM, DMARC) to minimize the chances of being throttled by ISPs like Comcast.
Marketer from Email Geeks, alex_brotman after discussion with Brian Godiksen, clarifies that retrying deferrals within a few minutes is fine, but trying to "run us over to get it through" is not recommended.
Email marketer from Stackoverflow explains that a 421 error indicates a temporary failure, and the sending server should retry delivery after a delay. They suggest checking server logs for more specific error details from Comcast.
Email marketer from SendGrid Blog recommends implementing a feedback loop with ISPs like Comcast to understand their delivery policies and address any issues promptly. Also, actively manage your subscriber list by removing inactive users.
Email marketer from Neil Patel's Blog advises focusing on building a clean email list, segmenting your audience, and personalizing your messages to improve engagement and reduce bounce rates, which can negatively impact your sender reputation and lead to throttling.
Marketer from Email Geeks explains that Comcast throttling is based on historical quantity/quality.
What the experts say3Expert opinions
Expert from Email Geeks explains the user is being told that they need to send slower and limit the number of connections they're making to Comcast's infrastructure.
Expert from Word to the Wise, Laura Atkins, suggests checking your domain and IP reputation by using various tools, and also making sure to authenticate your emails. Also check feedback loops to see what Comcast is saying about your mailings.
Expert from Spam Resource, John Levine, suggests that Comcast is known to aggressively block email and recommends contacting them directly to inquire about their policies and potential solutions.
What the documentation says4Technical articles
Documentation from Spamhaus explains that checking your IP address against blocklists is crucial. If your IP is listed, follow the delisting procedures outlined by the blocklist provider to improve deliverability and reduce deferrals.
Documentation from Comcast Postmaster Website explains that throttling (RL000003) indicates that your sending volume has exceeded established thresholds. They recommend reducing sending rates and the number of concurrent connections.
Documentation from Microsoft Support suggests checking your sender reputation, ensuring your IP address isn't blacklisted, and verifying your email authentication (SPF, DKIM, DMARC) records are properly configured to help avoid deferrals.
Documentation from RFC 5321 (Section 4.2.5) explains that a 4xx SMTP reply code (like 421) indicates a temporary failure. The sender should defer sending the message and try again later.