Why does an email report show an open when the mailbox is unknown?

Key findings

• Automated scanning: Recipient servers and security solutions frequently scan incoming emails for malicious content or phishing links. This pre-scanning involves loading the email's content, which triggers the tracking pixel and registers an 'open'.
• SMTP sequence: The Simple Mail Transfer Protocol (SMTP) transaction process involves stages such as RCPT TO (recipient verification) and DATA (content transfer). An 'open' can be registered during the DATA phase before a final hard bounce is returned for an invalid recipient.
• Proxy services: Email proxy services, like Gmail's image proxy, pre-load all images (including tracking pixels) to ensure privacy and optimize content, regardless of the recipient's validity.
• Data inaccuracy: These automated interactions can significantly inflate reported open rates, making them unreliable indicators of genuine human engagement.

Key considerations

• Distinguish human vs. machine opens: Modern email analytics tools should ideally differentiate between actual user engagement and automated scanning activity.
• Investigate bounce messages: Always review the precise bounce code and message from the recipient server. This information is crucial for understanding the exact reason for the delivery failure, even if an open was recorded.
• Review ESP reporting: Understand how your Email Service Provider classifies and reports these seemingly contradictory events. Some ESPs might log an open as long as the pixel loads, irrespective of a subsequent bounce.
• Focus on deeper engagement: Given the unreliability of open rates, prioritize metrics like clicks, conversions, and direct replies as more accurate indicators of subscriber engagement.

Key opinions

• Conflicting data: Many marketers are puzzled by reports that show an email was opened for a mailbox that is simultaneously marked as unknown.
• ESP classifications: The term 'unknown mailbox' can be a generic classification by the ESP for various bounce messages, rather than the raw error code from the receiving server.
• Incomplete bounce messages: Marketers often receive truncated or simplified bounce messages, making it difficult to diagnose the exact reason for non-delivery or the conflicting 'open' status.
• Timestamp discrepancies: The presence of both 'date bounced' and 'opened' timestamps for the same email can lead to confusion about the actual email journey.

Key considerations

• Verify reporting logic: Marketers should delve into how their sending platform defines and logs opens, particularly in scenarios involving automated scans or delayed bounces.
• Request full bounce details: Insist on obtaining the full SMTP bounce message, including the precise error codes, to understand the true delivery status.
• List hygiene: Even with reported opens, addresses leading to unknown mailbox bounces should be promptly removed from active lists to maintain sender reputation and reduce bounce rates.
• Rethink open metrics: Consider the limitations of open tracking due to machine interactions and focus more on actionable engagement metrics like clicks and conversions.

Key opinions

• Machine-triggered opens: The 'open' event is almost certainly triggered by a machine (e.g., recipient server scanner, anti-spam filter) interacting with the email's content, including the tracking pixel.
• Post-DATA bounce: The rejection message (e.g., 'user unknown') may come after the DATA phase of the SMTP transaction, allowing the server to scan the content and trigger the open pixel before sending the bounce.
• Bot impressions: Automated 'bot impressions' of tracking pixels can significantly skew reported open rates, as these do not represent genuine human engagement.
• Spam traps and blocklists: Some blocklists or spam traps may deliberately open emails to analyze their content, contributing to these false positives.

Key considerations

• Understand SMTP: A fundamental understanding of the SMTP transaction process is crucial for diagnosing such issues, particularly the order of commands like RCPT TO and DATA.
• Focus on real engagement: Experts advise shifting focus from raw open rates to more reliable metrics like clicks, conversions, and direct replies to gauge actual audience interest.
• Anticipate automated scans: Recognize that automated scanning is a standard practice and will impact reported opens, requiring adjustments to how these metrics are interpreted.
• Analyze full logs: Detailed log analysis, including the exact bounce message and timing of events, is necessary to differentiate between genuine and machine-triggered opens.

Key findings

• SMTP protocol: The fundamental SMTP transaction model allows for the transfer of email content (DATA command) before a definitive recipient status (such as 'unknown user') is returned after the message has been fully accepted for processing, as detailed in SMTP transaction process documentation.
• Security scanning: Many email providers, including Microsoft 365, employ advanced threat protection (ATP) and anti-phishing mechanisms that scan email content and links before delivering the message, which can trigger tracking pixels.
• Image proxying: Services like Google Image Proxy re-host and serve images from their own servers, often pre-fetching them automatically, irrespective of whether the end-user opens the email or if the mailbox is valid.
• Bounce reporting: Tools like Exchange Online's message trace report provide detailed logs that can help trace the journey of an email and differentiate between successful delivery and various bounce types.

Key considerations

• Interpreting bounce codes: Rely on standard SMTP error codes (e.g., 550 5.1.1 for unknown user) for definitive reasons behind delivery failures, rather than just generic descriptions.
• Understanding false positives: Documentation confirms that a portion of reported opens will be machine-generated, and this should be factored into campaign analysis and goal setting.
• Leverage diagnostic tools: Utilize built-in diagnostics and message tracing tools from email providers to gain granular insights into email flow and delivery outcomes.
• Refine engagement metrics: Aligning with industry best practices means moving beyond open rates as a sole measure of success, favoring more reliable engagement metrics for strategic decisions.