What does a TempFail suspected spam error mean for email delivery and how to troubleshoot it?

Key findings

• Misleading classification: ESPs often oversimplify or misclassify bounce messages, leading to inaccurate troubleshooting.
• Raw bounce messages: The actual deferral messages from the recipient's ISP are critical for understanding the true nature of the delivery issue.
• Permanent disguised as temporary: Many errors reported as TempFail are actually permanent failures, such as invalid recipient addresses or domain configuration issues.
• Spikes indicate issues: Even small increases in delivery error rates can signal underlying problems with data quality or sender reputation.

Key considerations

• Obtain raw bounce messages: Always request the full, unedited bounce messages from your ESP for accurate diagnosis.
• Verify ESP classification: Do not solely rely on your ESP's interpretation of bounce codes. Consult resources on SMTP codes and their meanings.
• Examine mailing list quality: Poor engagement or outdated email addresses are common causes for these errors.
• Monitor for subtle changes: Consistent monitoring of delivery metrics can help detect early warning signs before they escalate.

Key opinions

• ESP reporting limitations: Marketers find that ESPs sometimes provide generalized or inaccurate bounce classifications.
• Need for detailed information: The actual bounce message is essential for proper diagnosis, as ESP summaries are often insufficient.
• Impact on deliverability: Even seemingly temporary errors, if consistent, can indicate broader deliverability challenges and a risk to sender reputation.
• Data quality concerns: Many suspected spam issues are rooted in poor list hygiene or unengaged recipients.

Key considerations

• Request full bounce messages: Always insist on obtaining the detailed deferral messages directly from your Email Service Provider (ESP).
• Assess recent list acquisitions: Investigate new data sources if error rates correlate with list growth.
• Implement regular list hygiene: Proactively remove unengaged or invalid email addresses to maintain list quality and prevent issues listed in the email delivery failure causes.
• Cross-reference metrics: Look beyond a single metric like spam rate and check other indicators such as delivery error rate and overall domain reputation.

Key opinions

• ESP classifications are unreliable: ESPs often generalize bounce messages, making them unhelpful for accurate troubleshooting.
• Soft bounces are often hard: Errors like inbox full or no MX or A for domain are frequently permanent, despite being reported as temporary (soft bounces).
• Internal suppression: Bounce messages with [internal] often mean the sending Mail Transfer Agent (MTA) suppressed the message and did not attempt delivery.
• Data quality is critical: A significant portion of these delivery errors stems from invalid or unengaged email addresses, reinforcing the importance of list hygiene. Refer to Word to the Wise for more.

Key considerations

• Demand original deferral messages: Always seek the raw SMTP response codes from your ESP, as they provide accurate insights into the problem.
• Implement robust bounce management: Ensure your system effectively removes invalid addresses to prevent repeated failures that might lead to SMTP 550 errors.
• Address list churn: Understand that a certain bounce rate is natural due to recipient churn, but identify abnormal spikes.
• Distinguish soft vs. hard bounces: Proper classification of bounce types is crucial for effective list hygiene and improving deliverability. This can also prevent issues with DNS records.

Key findings

• 4xx vs. 5xx codes: 4xx codes denote temporary conditions (e.g., mail server busy), while 5xx codes signify permanent failures (e.g., invalid address, blacklisted sender).
• Retry mechanism: Mail servers are designed to retry delivery for 4xx errors over a period, as detailed in SMTP standards like Fastmail's explanation of bounces.
• Spam detection indication: A TempFail with suspected spam suggests the recipient mail server flagged the message due to content, sender reputation, or other policy violations.
• DNS records: Errors like no MX or A for domain indicate permanent failures due to incorrect or missing DNS mail routing records.

Key considerations

• Adherence to RFCs: Understanding SMTP RFCs (e.g., RFC 5321) helps in accurately interpreting complex bounce messages and ensuring protocol compliance.
• Proper bounce handling: Implement system logic to distinguish between retriable 4xx errors and unretriable 5xx errors for efficient queue management.
• Sender reputation feedback: Mail server responses are direct feedback on your sender reputation. A TempFail suspected spam signal indicates a need to review your email content and sending practices.
• DNS configuration: Regularly verify your domain's MX and A records for correct setup to prevent permanent delivery failures. This is also important for email authentication.