What to do when getting 'not our customer' hard bounce message from Comcast?
Matthew Whittaker
Co-founder & CTO, Suped
Published 17 Jun 2025
Updated 17 Aug 2025
7 min read
Receiving a "not our customer" hard bounce message from Comcast (or Xfinity, as it's often known) can be a frustrating experience for any sender. This specific error, often accompanied by a 550 5.1.1 Not our customer or 550 5.2.0 Not our customer SMTP code, indicates a permanent failure. Essentially, the Comcast mail server is telling you that the recipient email address does not exist on their system.
This isn't a temporary issue like a full mailbox or a transient network problem, which would typically result in a soft bounce. A hard bounce signifies that the email address is permanently invalid or has been deactivated. My immediate concern when I see this is usually about the accuracy and hygiene of my email list.
While such bounces are a clear signal to remove the invalid address from your list, there can sometimes be nuances, especially with a major ISP like Comcast (or Xfinity). This guide will walk you through understanding why this happens, how to investigate, and what steps to take to resolve and prevent it in the future.
Understanding the 'not our customer' bounce
When you encounter a "not our customer" bounce, it's essentially Comcast's way of stating that the mailbox you're trying to reach does not exist within their system. This is a definitive rejection. Unlike other bounce types that might indicate a temporary problem or content filtering, this error specifically points to the invalidity of the recipient address itself.
It's crucial to distinguish this from a soft bounce. Soft bounces, such as a full inbox or a temporary server issue, mean the email couldn't be delivered at that moment but might be delivered later. A hard bounce, on the other hand, means the email will never be delivered to that specific address. Continued attempts to send to a hard-bounced address can severely damage your sender reputation and lead to your IP or domain being added to a blacklist (or blocklist).
Hard bounces vs. soft bounces
Hard bounce: Indicates a permanent delivery failure. Examples include "address doesn't exist" or "domain invalid." These addresses must be removed immediately.
Soft bounce: Indicates a temporary delivery issue. Examples include a full inbox, server downtime, or a message too large. You can retry sending to these addresses later.
The message "not our customer" is quite explicit. It means that Comcast (or Xfinity) does not recognize the local part of the email address (the part before the @ symbol). This could be due to several reasons, which we will explore further.
Common causes of this hard bounce
While the bounce message itself points to an invalid address, understanding the underlying causes can help you prevent future occurrences. One of the most common reasons is simply a typo in the email address. Even a single misplaced character can lead to this error.
Another frequent cause is that the email account has been deactivated or closed. This is particularly common with residential ISPs like Comcast, where users might change providers or close their accounts. Over time, email lists naturally decay, and accounts become obsolete.
Less common, but still possible, are ISP-side issues. There have been instances where ISPs, including Comcast, experience temporary glitches or migrations (such as Comcast's move to Yahoo! Mail) that can cause legitimate addresses to temporarily bounce with this error. While rare, it's worth considering if you see a sudden, widespread spike in these bounces.
Common causes
Typographical errors: The recipient's email address was entered incorrectly.
Deactivated account: The Comcast user closed their account or stopped using the email address.
Fake or invalid address: The email address was never valid to begin with (e.g., a spam trap).
Less common causes
ISP-side glitch: Temporary system errors at Yahoo (who handles Comcast mail) or Comcast itself.
Migration issues: Problems arising from email service migrations, potentially causing temporary misclassifications.
Steps to investigate and verify
When you receive a "not our customer" bounce, your first step should always be to verify the recipient's email address. Double-check for any typos. If it's a critical contact, consider reaching out via an alternative channel to confirm their current email address. For a large list, manual checking isn't feasible, so you'll rely on bounce reports from your Email Service Provider (ESP).
Next, examine the context of the bounce. Is it an isolated incident, or are you seeing a sudden spike in these bounces, particularly to Comcast addresses? A widespread issue might suggest a temporary problem on the ISP's side, as has been observed historically with Comcast bounces around specific dates. However, if it's consistently occurring for individual addresses, the problem is most likely with the address itself.
Example bounce message
550 5.1.1 <user@comcast.net>... User unknown
For a comprehensive understanding of delivery issues, review your ESP's bounce reports carefully. They often provide the exact SMTP error code and message, which gives valuable clues. Also, cross-reference these bounces with recent engagement data for those contacts. If an address that was recently active suddenly hard bounces with "not our customer," it strengthens the case for an ISP-side anomaly. For more general troubleshooting of bounce messages, you can review how to troubleshoot email bounce messages.
Remediation and prevention strategies
The primary remediation strategy for a "not our customer" hard bounce is immediate removal of the affected email address from your mailing list. Sending to invalid addresses harms your sender reputation, reduces inbox placement, and can lead to blacklisting or throttling by ISPs like Comcast. Most reputable ESPs will automatically suppress these addresses, but it's important to understand why this happens and take preventative measures.
To prevent future hard bounces, focus on list hygiene and robust opt-in practices. Implement a double opt-in process for new subscribers to ensure they are providing valid and active email addresses. Regularly clean your email list by removing inactive subscribers and those who have previously hard bounced. For further insights on why emails fail and how to improve deliverability, explore guides on why your emails fail.
Best practices for preventing hard bounces
Implement double opt-in: Confirm subscriber intent and email validity from the start.
Regular list cleaning: Remove unengaged subscribers and bounced addresses promptly.
Monitor bounce reports: Pay close attention to error codes and patterns to address issues quickly.
Finally, ensure your email authentication protocols (SPF, DKIM, and DMARC) are correctly configured. While these don't directly prevent a "not our customer" bounce, they are vital for overall deliverability and sender reputation, helping you avoid other types of rejections and blacklisting (or blocklisting). A well-configured setup helps ISPs like Comcast trust your sending domain. Learn more in our simple guide to DMARC, SPF, and DKIM.
Views from the trenches
Best practices
Always implement double opt-in to verify new subscriber email addresses and reduce invalid sign-ups.
Regularly clean your email lists to remove hard bounces and unengaged subscribers.
Monitor your bounce reports diligently, especially for specific ISPs like Comcast, to identify unusual patterns.
Maintain strong sender authentication (SPF, DKIM, DMARC) to build trust with mailbox providers.
Segment your audience and send relevant content to minimize unengagement and subsequent list decay.
Common pitfalls
Ignoring "not our customer" hard bounces and continuing to send to invalid addresses, which harms sender reputation.
Failing to implement proper list hygiene, leading to a build-up of invalid or inactive contacts.
Not monitoring for sudden spikes in bounce rates from specific ISPs, which could indicate a systemic issue.
Overlooking the distinction between hard bounces and soft bounces, and treating them all the same.
Not having a clear process for re-engaging or confirming potentially valid bounced email addresses.
Expert tips
If you see a sudden, widespread increase in "not our customer" bounces from a specific ISP, cross-reference with other senders' experiences (e.g., industry forums) to check for a temporary ISP glitch.
For critical contacts who hard bounce, try reaching out via an alternative channel to confirm their current email address before removing them permanently.
Automate the removal of hard-bounced addresses through your ESP to protect your sender reputation proactively.
Consider using a real-time email verification service at the point of sign-up to catch invalid addresses before they even enter your list.
Analyze engagement data to identify subscribers who are consistently not opening or clicking, as these might be dormant or inactive accounts that could eventually turn into hard bounces.
Marketer view
A marketer from Email Geeks says they observed an increase in unclassified soft bounces from Comcast email addresses, suggesting a potential anomaly on Comcast's side.
March 15, 2024 - Email Geeks
Expert view
An expert from Email Geeks says their customers have not experienced soft bounces but did note typical throttling from Comcast, indicating different experiences among senders.
March 15, 2024 - Email Geeks
Final thoughts on Comcast hard bounces
Dealing with "not our customer" hard bounces from Comcast requires a systematic approach. While it's typically a clear sign of an invalid email address, it's always worth a quick investigation, especially if you see a sudden, widespread increase in these errors.
My advice is to prioritize list hygiene and implement robust opt-in processes. By keeping your email lists clean and ensuring you're sending to engaged subscribers, you'll naturally minimize these types of bounces and protect your sender reputation, ensuring your messages reach legitimate inboxes.
Comcast (or Xfinity, as it's often known) can be a frustrating experience for any sender. This specific error, often accompanied by a 550 5.1.1 Not our customer or 550 5.2.0 Not our customer SMTP code, indicates a permanent failure. Essentially, the Comcast mail server is telling you that the recipient email address does not exist on their system.
Xfinity). This guide will walk you through understanding why this happens, how to investigate, and what steps to take to resolve and prevent it in the future.
Yahoo (who handles Comcast mail) or Comcast itself.
