What to do when getting 'not our customer' hard bounce message from Comcast?

Key findings

• Anomalous Bounces: On specific dates, such as March 12, 2024, Comcast may have experienced temporary glitches, leading to 'not our customer' bounces for otherwise valid email addresses. These are typically unusual occurrences.
• Bounce Classification: Depending on your Email Service Provider (ESP), the '550 5.1.1 Not our customer' message might be classified as either a soft or a hard bounce, even though it indicates a permanent delivery failure. Typically, such errors are hard bounces.
• Temporary vs. Permanent: While a 'not our customer' message generally signifies a permanent bounce (indicating the address doesn't exist), there can be temporary server-side issues that cause legitimate addresses to bounce incorrectly.
• Comcast-Specific Issues: Comcast (Xfinity) is known for rigorous filtering and occasionally has localized issues or throttling, which can impact email deliverability, leading to varied bounce responses.

Key considerations

• Verify Bounce Message: Always obtain the exact bounce message from your ESP to determine if it aligns with the '550 5.1.1 Not our customer' or '550 5.2.0 Not our customer' error, as this helps in accurate diagnosis.
• Recipient Status: For the specific date mentioned (March 12), an expert suggested ignoring these bounces, meaning the recipient's status might not need immediate update. However, typically, this bounce code warrants removing the address from your active list.
• List Hygiene: Maintaining a clean email list is crucial. Regularly remove hard bounced addresses to protect your sender reputation and improve overall deliverability. This includes understanding what to do when a user's email hard bounces.
• Monitor Deliverability: Keep a close eye on your bounce rates, especially to large ISPs like Comcast. Sudden spikes can indicate broader issues with your sending practices or ISP-specific filtering. Understanding how to resolve Comcast email rejections and throttling is important.

Key opinions

• Inconsistent Bounce Types: Some marketers reported hundreds of soft bounces, while others experienced hard bounces with the exact same '550 5.1.1 Not our customer' message around the same time period.
• ESP Classification Differences: There's an open question about whether different ESPs might classify identical bounce messages (e.g., 'not our customer') as either soft or hard, leading to confusion.
• Validity Concerns: Many marketers expressed strong belief that the bounced addresses were indeed valid, having successfully emailed them just days prior.
• Volume Sensitivity: Sending large volumes of emails, even legitimate ones, to Comcast recipients can sometimes trigger their spam filters or advanced security measures, leading to blocks or bounces.

Key considerations

• Review ESP Logs: Marketers should always ask their ESP for the exact bounce message and context to understand if the issue is a temporary anomaly or a genuine hard bounce. Tracking email activity is essential.
• Temporary Suppression: If unsure, or if it aligns with a known ISP-side glitch, temporarily suppressing these addresses for a few campaigns might be a cautious approach to avoid further reputation damage.
• Comcast Throttling: Be aware that Comcast frequently throttles email sends, which might sometimes manifest as high soft bounce rates or delayed delivery without explicit bounce messages. This is similar to issues with other major ISPs.
• Contacting Recipients: For critical contacts, consider reaching out to them through alternative channels to re-confirm their email address or encourage them to re-opt-in, especially if you suspect the bounce was erroneous.

Key opinions

• Context Matters: Experts advise that 'not our customer' bounces from a specific date (e.g., March 12) might be anomalous and should be ignored, rather than triggering immediate list removal.
• General Rule for Hard Bounces: However, the prevailing expert consensus is that, generally, 'not our customer' messages (including '550 5.1.1 Not our Customer' and '550 5.2.0 Not our Customer') should be treated as permanent hard bounces.
• List Hygiene Importance: Maintaining a clean email list is paramount for minimizing bounces and ensuring a good sender reputation, as emphasized by deliverability experts.
• ISP Filtering Sophistication: Comcast and other large ISPs use advanced filtering systems that can sometimes misclassify legitimate emails or implement temporary blocks, contributing to unexpected bounce rates.

Key considerations

• Investigate Anomalies: When sudden, widespread hard bounce events occur for a major ISP like Comcast, experts suggest investigating potential server-side issues at the ISP or a rapid shift in sender reputation. Learn how to understand your email domain reputation.
• Remove Invalid Addresses: Unless there's a specific, confirmed anomaly (like the March 12 Comcast event), addresses generating 'user unknown' or 'not our customer' errors should be promptly removed from your active mailing lists. This helps in processing reputation-based bounces.
• Monitor Bounce Logs: Close monitoring of bounce logs and a deep understanding of bounce codes are essential for effective diagnosis of deliverability issues.
• Engage with ISP: Consider contacting the ISP's postmaster or abuse desk (e.g., Comcast's Customer Security Assurance) for insights into specific blocking reasons, although direct responses are not always guaranteed.

Key findings

• 550 SMTP Error: RFC 5321 defines the '550' SMTP error as 'Requested action not taken: Mailbox unavailable,' typically meaning the mailbox could not be found or access was denied.
• 5.1.1 Status Code: RFC 3463 clarifies that a '5.1.1' status code indicates 'Bad destination mailbox address,' meaning the specified mailbox does not exist at the receiving server.
• 5.2.0 Status Code: RFC 3463 also describes '5.2.0' as 'Mailbox frozen; cannot accept mail,' implying a temporary inability for the mailbox to receive mail, though it might become available later.
• Hard Bounce Definition: Hard bounces are generally permanent delivery failures, most often due to non-existent email addresses, and should lead to immediate removal from mailing lists.

Key considerations

• Accurate Interpretation: Properly interpret SMTP error codes and enhanced status codes to distinguish between temporary issues and permanent delivery failures. This is essential for effective bounce management.
• List Hygiene Automation: Implement automated processes to remove hard bounced addresses from your mailing lists promptly. This protects your sender reputation and improves overall deliverability rates.
• Sender Reputation Impact: A consistently low bounce rate is crucial for maintaining a good sender reputation. High bounce rates signal poor list quality to ISPs, potentially leading to blocklisting or reduced inbox placement.
• Recipient Verification: While some mail servers perform recipient verification that can cause temporary rejections for valid addresses (if the server is overloaded), 'not our customer' generally bypasses this to indicate non-existence.