Summary
The consensus from experts, marketers, and technical documentation is that an email report can show an 'open' for an unknown mailbox due to automated processes triggered before the email's final delivery status is determined. Spam filters, AV scanners, security systems, and email client pre-fetch mechanisms load images (including tracking pixels), registering an open event even if the mailbox is invalid or the email never reaches a human recipient. These automated actions occur before the bounce message is generated, and factors like Apple's Mail Privacy Protection further skew open rate accuracy. The key is that tracking pixels measure machine interactions, not necessarily human engagement, leading to discrepancies between reported opens and actual views.
Key findings
- Automated Scanning: Spam filters, AV scanners, and security systems proactively scan emails, triggering tracking pixels.
- Image Pre-fetching: Email clients often pre-fetch images for security or rendering, inflating open rates.
- Asynchronous Events: Open tracking is recorded as soon as the pixel loads, independent of eventual delivery or bounce status.
- Privacy Protection Impact: Features like Apple Mail Privacy Protection automatically load tracking pixels, artificially inflating open rates.
- Machine vs. Human: Tracking pixels measure machine interactions, not necessarily genuine human engagement.
- Unreliable Metric: Open-tracking has become an unreliable measure of user behavior due to evolving anti-spam algorithms. It will be more difficult in the future to gauge email success.
Key considerations
- Re-evaluate KPIs: Rely less on open rates; instead, focus on more reliable engagement metrics like click-through rates, conversions, and website visits.
- Segment Email List: Maintain clean email lists and practice segmentation.
- Interpret Data Critically: Be aware that open rates are often inflated, and interpret data with caution.
- Analyze Rejection Messages: Examine rejection and bounce messages for additional context to understand why emails aren't delivered.
- Consider User Settings: Remember that some users disable image loading, making open tracking inherently incomplete.
What email marketers say
13 marketer opinions
The consensus is that an email report showing an open for an unknown mailbox is primarily due to automated processes, such as security scans, spam filters, and image pre-fetching by email clients. These systems trigger tracking pixels before the email ever reaches a valid recipient, resulting in a false 'open' signal. Privacy features like Apple's Mail Privacy Protection also contribute to inflated open rates. Additionally, the timing of bounce messages and open tracking requests can be asynchronous, with the open recorded before the mailbox validity is determined.
Key opinions
- Bot Activity: Security scanners, spam filters, and other bots often trigger open tracking pixels by loading images.
- Privacy Changes: Privacy features like Apple Mail Privacy Protection inflate open rates.
- Asynchronous Events: Open tracking can be recorded before a bounce message is generated, leading to discrepancies.
- Pre-fetch Mechanisms: Email providers proactively download images, causing opens before the intended recipient sees the email.
- Machine Interactions: Tracking pixels measure machine interactions, not necessarily human ones.
- Invalid Metrics: Open-tracking has become an unreliable metric for user behavior due to evolving anti-spam policies and algorithms. Open Rate is becoming an invalid measure of email campaign success.
Key considerations
- Rethink Open Rates: Rethink Key Performance Indicators (KPIs) as open rates might be misleading metrics. Consider using other KPIs. Rely less on open rates and more on clicks, conversions, and other engagement metrics.
- Segment & Clean Lists: Maintain clean email lists. Regularly clean email lists to ensure deliverability success.
- Interpret Data Cautiously: Understand that opens don't always equate to human engagement and consider the context of your email environment.
- Spam Traps: Be aware that opens caused by spam traps are asynchronous and can be detected after tracking has already fired.
Marketer view
Email marketer from Litmus shares that open rates are becoming less reliable due to privacy changes and how email clients handle image loading. Apple's Mail Privacy Protection, for example, inflates open rates by loading tracking pixels for all emails, regardless of whether the recipient interacts with them. Similarly, security scans can trigger opens even when the mailbox is invalid.
13 Apr 2024 - Litmus
Marketer view
Email marketer from Quora explains that, some users have images automatically turned off in their email settings. In those cases, open tracking will not be accurate, but this also applies to bot opens because the images are loaded by the server and not by the user.
14 Dec 2022 - Quora
What the experts say
3 expert opinions
Experts agree that an email can register as 'opened' despite an unknown mailbox primarily due to automated processes. Spam filters, AV scanners, and even preview panes can access the email and load images (including tracking pixels) before the email's validity is fully assessed. This occurs when a mail server sees the request for an image, considers the message read, and the mailbox is flagged as invalid afterwards.
Key opinions
- Automated Scans: Spam filters and AV scanners scan emails before they reach the intended recipient.
- Image Loading: The loading of images (tracking pixels) by these automated systems triggers open tracking.
- Asynchronous Validation: Mailbox invalidation might occur after the open tracking pixel has already fired.
- Limited Information: The 'unknown mailbox' message itself provides limited context; the rejection message offers more details.
Key considerations
- Analyze Rejection Messages: Review rejection messages for additional context beyond the 'unknown mailbox' notification.
- Account for Automated Opens: Recognize that open rates might be inflated due to automated processes and adjust analysis accordingly.
- Focus on Engagement: Consider shifting focus to other metrics like click-through rates to better understand recipient engagement.
Expert view
Expert from Email Geeks suggests that the 'unknown mailbox' message itself doesn't reveal much. The rejection message, however, provides more context and aligns with Daniel's suggestion about server scanning.
19 Jan 2024 - Email Geeks
Expert view
Expert from Word to the Wise explains that spam filters often scan emails before delivery. When the spam filter scans the email, it requests the image to be displayed, which will increment the open tracking counter even when it never reaches the end-user.
1 Aug 2024 - Word to the Wise
What the documentation says
5 technical articles
Technical documentation indicates that 'opens' can be recorded even for unknown mailboxes due to email client pre-fetching of images and security systems scanning email content. Email clients and servers often load images automatically for security purposes, triggering tracking pixels. This can occur before the email address is validated or a Non-Delivery Report (NDR) is generated, leading to an 'open' event despite the ultimate delivery failure. SMTP servers and security filters might process the email, including loading tracking pixels, before a 'mailbox unknown' error is communicated.
Key findings
- Image Pre-fetching: Email clients often pre-fetch images automatically, registering an open before user interaction.
- Security Scanning: Security systems scan emails for malware/spam, potentially triggering tracking pixels.
- Asynchronous Bounces: Bounces occur separately from the initial email processing and pixel firing.
- Early Processing: Email processing (including image loading) can happen before mailbox validation.
Key considerations
- Account for Pre-fetching: Recognize that email client image pre-fetching can inflate open rates.
- Understand Security Impact: Be aware of security system interactions when analyzing open tracking data.
- Consider Bounce Timing: Factor in that bounces might be delayed and not reflect the initial processing of the email.
Technical article
Documentation from Mailchimp explains that discrepancies between 'opens' and recipient data can occur because of email clients pre-fetching images. Many email clients load images automatically as a security feature, registering an open even if the recipient hasn't actually viewed the email. This can happen even with invalid or non-existent email addresses if the security system accesses the email.
11 May 2023 - Mailchimp
Technical article
Documentation from RFC 5321 explains that SMTP servers can provide various response codes. A 'mailbox unknown' error typically occurs during the SMTP transaction. However, a security filter might still access and process the email content (including tracking pixels) *before* this rejection is communicated back to the sender.
23 Dec 2022 - RFC 5321
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