Why are emails hard bouncing then opening and clicking links later?

Key findings

• Pre-delivery scanning: Many email security systems and spam filters (particularly those for corporate or government domains) scan incoming emails for malicious content, including opening links and loading images, before the email is delivered to the recipient’s inbox. This automated activity can register as an open or click, even if the email ultimately hard bounces.
• Delayed bounce reporting: There can be a delay between the actual delivery attempt (and subsequent scanning) and when the hard bounce notification (Delivery Status Notification or DSN) is processed and reflected in your sending platform's reports. This delay can lead to the impression that an email was engaged with after bouncing.
• Mis-categorization by ESPs: Some Email Service Providers (ESPs) may occasionally mis-categorize bounce types, or their reporting systems may struggle to reconcile automated engagement with a later hard bounce. This might also occur with emails that are marked as read despite bouncing.
• Variable Envelope Return Path (VERP) issues: If VERP is used, some platforms might misinterpret spam or junk mail sent to a bounce address as a hard bounce for a legitimate campaign, leading to confusing data.
• Persistent problematic addresses: Seeing multiple hard bounces over months on the same address, sometimes with intermittent engagement, often points to a fundamental issue with that email address itself or how it's being handled in your list management process.

Key considerations

• Investigate bounce reasons: Delve into the raw SMTP bounce reasons to understand the true cause of the hard bounce. This can differentiate between a non-existent mailbox and a temporary block that was later bypassed by a security scanner.
• Review ESP bounce logic: Understand how your ESP classifies and reports bounces. Some platforms may have specific definitions for hard and soft bounces that could lead to these discrepancies.
• Automated engagement vs. human: Recognize that opens and clicks on hard bounced emails are almost certainly automated system activity, not human engagement. Don't let these metrics skew your understanding of list health or campaign performance.
• List hygiene: Implement robust list hygiene practices to promptly remove hard bounced addresses to protect your sender reputation. Continuing to send to hard bounced addresses can negatively impact your deliverability across all mailbox providers.

Key opinions

• Security scanner activity: Many marketers believe that automated content and link scanners are a primary driver of opens and clicks on emails that later hard bounce. These scanners pre-fetch content, creating engagement data before the email is definitively rejected.
• Delayed delivery decisions: Some recipient servers, particularly those with advanced filtering, may initially accept an email but then reject it after a deeper scan (e.g., after the DATA command in SMTP). Engagement might occur during this temporary holding period.
• ESP reporting quirks: Marketers frequently suspect that their ESPs might be miscategorizing bounce types or have reporting systems that struggle to accurately reconcile the timeline of a bounce versus engagement, leading to confusing metrics.
• VERP complexities: The use of VERP (Variable Envelope Return Path) can complicate bounce handling, potentially leading to extraneous spam hitting bounce addresses being incorrectly logged as hard bounces for legitimate campaigns.
• Bounce management issues: Persistent hard bounces on the same address often indicate underlying problems with a client's list hygiene or the effectiveness of their bounce management processes.

Key considerations

• Raw bounce data: Marketers recognize the importance of accessing raw bounce reasons and SMTP codes from their ESPs, as this granular data is crucial for understanding the true nature of delivery failures.
• Advanced reporting needs: There's a desire for more sophisticated bounce reporting from ESPs, which would detail at what stage of the SMTP transaction an email was rejected. This would help clarify misleading engagement metrics from automated systems.
• Balancing data access: Marketers understand that while more detailed bounce information is valuable, ESPs face a challenge in providing it widely without overwhelming clients with technical complexity and increasing support demands.
• Reviewing data from other sources: To gain a full picture, marketers often cross-reference ESP data with other analytics tools or internal logs to identify patterns of artificial engagement.

Key opinions

• SMTP transaction stages: Experts highlight that rejections can occur at various stages of the SMTP conversation (e.g., HELO, MAIL FROM, RCPT TO, DATA). If a rejection happens after the DATA command, the recipient server has the full message, allowing security scanners to trigger opens/clicks before the bounce.
• Content analysis before acceptance: Many mail servers perform deep content analysis, including URL scanning and sandbox detonation, before formally accepting a message or delivering it to the inbox. This process generates activity that can be mistaken for human engagement.
• Transient vs. permanent errors: Distinguishing between temporary (4xx) and permanent (5xx) SMTP bounce codes is critical. A temporary rejection might see subsequent delivery attempts, during which automated engagement could occur, even if the eventual outcome is a hard bounce (e.g., after multiple retries fail).
• Spoofing and backscatter: In some cases, seeing activity from a hard bounced address can be related to email backscatter, where misconfigured systems send non-delivery reports (NDRs) to spoofed sender addresses, potentially triggering spurious activity.
• Platform reporting logic: ESPs and mailing platforms interpret and present bounce data in various ways. Discrepancies between the raw DSNs and the simplified reports can lead to confusion when automated clicks are present.

Key considerations

• Focus on DSN analysis: Rely on the Delivery Status Notifications (DSNs) and the specific SMTP error codes received to determine if a bounce is truly permanent. These provide the most accurate technical reason for non-delivery.
• Impact on sender reputation: Even if opens/clicks are recorded, repeatedly sending to hard bounced addresses (which signals a non-existent user) negatively affects your sender reputation and could lead to blocklisting.
• Automated engagement vs. human behavior: It's crucial to differentiate between automated security scans (which might register as opens/clicks) and genuine human interaction. Hard bounces should still lead to list removal regardless of these phantom metrics.
• Proactive list cleaning: Regularly cleaning your email lists to remove hard bounces is paramount for maintaining high deliverability rates. This reduces the chances of hitting spam traps or being flagged by mailbox providers.

Key findings

• SMTP response codes: RFC 5321 (Simple Mail Transfer Protocol) defines 4xx (transient negative completion) and 5xx (permanent negative completion) reply codes. A hard bounce is typically a 5xx code, indicating a permanent failure.
• Delivery status notifications (DSNs): RFC 3464 describes DSNs, which are standardized bounce messages. These include a status field that can differentiate between a permanent fatal error (e.g., 5.1.1 Bad destination mailbox address) and a persistent temporary failure.
• Mail server behavior: Mail servers often perform pre-delivery content and link analysis in a sandbox or temporary holding queue. This allows them to identify threats before committing the message to the inbox, and this scanning can register as an open or click.
• Order of operations: Engagement tracking (opens via pixel, clicks via URL rewrite) occurs during or immediately after the email content is received by the recipient's system. If a hard bounce decision is made later in the processing chain, the engagement data may precede the bounce record.
• Automated email processing: Security gateways and anti-spam solutions are designed to actively interact with email content to assess risk. This automated interaction is not human engagement and should be distinguished from legitimate user activity.

Key considerations

• Understand DSN structure: Familiarize yourself with the components of a DSN, including the original recipient, reporting MTA, and specific diagnostic code, to accurately diagnose bounce reasons.
• Interpret SMTP codes: Properly interpreting 4xx (transient) versus 5xx (permanent) SMTP codes is fundamental to distinguishing between a temporary issue that might resolve and a permanent failure requiring list removal.
• Automated vs. human engagement: Technical documentation on email security products often describes how they scan and interact with email content, confirming that opens/clicks on bounced emails are likely automated and not genuine user actions.
• Platform-specific documentation: Consult your ESP's documentation regarding their bounce categorization and reporting logic, as these can vary significantly and influence how hard bounces with engagement are displayed.