Why are emails being delivered slowly to Comcast on Cyber Monday?
Michael Ko
Co-founder & CEO, Suped
Published 14 Jul 2025
Updated 17 Aug 2025
6 min read
Cyber Monday, a cornerstone of the holiday shopping season, typically brings an avalanche of marketing emails. For many senders, it's a critical day to connect with customers and drive sales. However, this past Cyber Monday, I observed significant delays in email delivery to Comcast recipients, a situation echoed by others in the email community. SMTP tests to mx1.mxge.comcast.net and mx2.mxge.comcast.net were consistently timing out after 15 seconds, indicating a deeper issue than simple network congestion.
While mail was queuing up rather than bouncing, the severely slow delivery posed a significant challenge for time-sensitive campaigns. Similar, though less severe, slowdowns were also noted with Yahoo and AOL during the Black Friday weekend, but Comcast's performance on Cyber Monday was exceptionally poor, resembling a slow drip. This experience highlights the critical need to understand what causes such delays, especially from major internet service providers like Comcast (Xfinity).
Understanding the nature of the problem
The observed timeouts and queuing suggest a confluence of factors, primarily overwhelming volume on a critical day. ISPs like Comcast employ sophisticated systems to manage incoming email traffic, including spam filters, greylisting, and throttling mechanisms. When the volume of legitimate email spikes dramatically, as it does on Cyber Monday, these systems can become overloaded, leading to delays.
One primary reason for such widespread delays during peak events is email throttling. This is when an email provider intentionally slows down the rate at which it accepts emails from a particular sender. While it's a common practice to prevent spam and manage server load, aggressive throttling can severely impact deliverability during high-volume periods. Comcast (and other major mailbox providers) use a variety of signals, including sender reputation, to determine how much mail to accept at any given time.
The “Unable to connect after 15 seconds” message indicates that the connection attempt itself is timing out, suggesting that Comcast's mail servers are either too busy to respond or are actively rejecting connection attempts from certain IPs, rather than just delaying the receipt of mail. This differs from a simple queue where the connection is made but mail delivery is slow. It points to a bottleneck at the very initial stage of the SMTP handshake.
Why do email delays occur?
Beyond the sheer volume of emails, several underlying factors contribute to delays, especially with a major ISP like Comcast. During peak mailing seasons like Cyber Monday, all these factors are amplified.
Sender reputation: ISPs heavily rely on your sender reputation. A poor reputation, due to high spam complaints, bounces, or sending to stale lists, can lead to aggressive throttling or outright blacklisting (also known as blocklisting).
Network congestion: Just as internet speeds can slow down due to high traffic, email networks also experience congestion. Cyber Monday's email volume puts immense strain on the infrastructure of major providers. Checking Xfinity outage reports can provide insights into broader network issues impacting email delivery.
Content and domain issues: Certain email content, subject lines, or even URLs within your emails can trigger spam filters, leading to delays or direct-to-spam placement. Your domain’s age, history, and perceived trustworthiness also play a role.
Authentication failures: Incorrect or missing email authentication records (SPF, DKIM, DMARC) can significantly hinder deliverability, as ISPs struggle to verify the legitimacy of your emails. This is a common reason for Comcast to reject emails.
I’ve seen instances where Comcast specifically flags emails from servers that exhibit patterns characteristic of spam. This can lead to various error messages, including sender rejections, or contribute to slow delivery as the system scrutinizes the incoming mail. It’s a delicate balance for mailbox providers, as they aim to protect their users without hindering legitimate communication.
Diagnosing and addressing deliverability issues
When facing slow delivery, especially during high-stakes periods, swift diagnosis is crucial. Here are key areas to investigate and actions to take:
Common causes
High volume: Sending significantly more emails than usual.
Poor sender reputation: High bounces, spam complaints, or sending to invalid addresses.
Missing authentication: SPF, DKIM, or DMARC not correctly configured.
Blocklisting: Your IP or domain appears on a public or private blacklist.
Mitigation strategies
Warm up IPs: Gradually increase sending volume, especially with new IPs.
Monitor sender reputation: Keep an eye on metrics like bounce rate and spam complaints.
Verify authentication: Ensure SPF, DKIM, and DMARC records are correctly set up.
Check blocklists: Regularly check if your IPs or domains are on any email blocklists.
Key troubleshooting steps
Review logs: Analyze your mail server logs for specific error codes or messages when sending to Comcast. Look for messages beyond just connection timed out.
Check blocklist status: Use a blocklist checker to confirm your sending IP or domain isn't listed, as this can cause immediate rejections or significant delays.
Reduce sending rate: If you notice queues building up for Comcast, try temporarily reducing your sending rate to them. This can help prevent further throttling and allow existing mail to clear.
Proactive measures for future peak events
To prevent similar slow delivery issues, especially during future peak seasons or to major ISPs like Comcast, proactive measures are essential. It's not just about reacting to problems, but building a robust and resilient email program.
Maintain high sender reputation: This is the cornerstone of good deliverability. Continuously clean your lists, remove inactive subscribers, and monitor engagement to minimize bounces and spam complaints.
Segment your sending: For large sends, consider segmenting your audience and staggering your sends. This helps distribute the load and prevents hitting throttling limits too quickly, thereby preventing email throttling and delays.
By understanding the potential pitfalls and implementing these preventative measures, you can significantly improve your chances of reaching the inbox, even during the most demanding email seasons.
Views from the trenches
Best practices
Always maintain a high sending reputation by keeping your email lists clean and engaged.
Implement and verify SPF, DKIM, and DMARC records to authenticate your email. This builds trust with ISPs and reduces the likelihood of throttling or blocking.
Gradually increase your sending volume, especially during peak periods, to avoid overwhelming ISPs and triggering their spam filters.
Monitor your mail server logs closely for any error messages or connection timeouts, as these are early indicators of potential issues.
Common pitfalls
Sending a large volume of emails without proper IP warming, leading to sudden throttling or rejections.
Neglecting email list hygiene, which results in high bounce rates and spam complaints, damaging sender reputation.
Failing to implement or correctly configure email authentication protocols, making your emails appear suspicious.
Not monitoring deliverability metrics, causing delays or blockages to go unnoticed until open rates plummet.
Expert tips
Consider using multiple IPs or different sending domains for various email streams (e.g., transactional vs. marketing) to distribute risk.
Engage with ISP postmaster sites for specific guidelines and to report any persistent issues you observe.
Review your email content for anything that might trigger spam filters, such as excessive links, deceptive language, or poor formatting.
Be prepared for unexpected delays during major sales events, as even legitimate mail can be impacted by network congestion.
Marketer view
Marketer from Email Geeks says they also saw slow delivery to Comcast, confirming it was not an isolated incident for them.
2023-11-28 - Email Geeks
Expert view
Expert from Email Geeks says high email volume on Cyber Monday likely caused significant network congestion, leading to
2023-11-28 - Email Geeks
Key takeaways for reliable email delivery
Email deliverability is a complex dance, especially during high-traffic periods like Cyber Monday. The slow delivery issues to Comcast on this crucial day were a stark reminder that even with perfectly crafted emails, external factors can heavily influence your campaign's success.
Prioritizing sender reputation, implementing proper authentication, and continuously monitoring your email performance are not just best practices, but necessities. By being proactive and responsive, you can minimize the impact of such delays and ensure your messages reach their intended recipients, even when ISPs like Comcast are under immense pressure.
