Why does an email report show an open when the mailbox is unknown?
Matthew Whittaker
Co-founder & CTO, Suped
Published 2 Jun 2025
Updated 16 Aug 2025
7 min read
It can be incredibly confusing to see an email report that indicates an email was "opened" yet simultaneously states that the "mailbox is unknown." This seemingly contradictory data point often leaves email marketers and deliverability professionals scratching their heads, wondering how a non-existent mailbox could possibly register an open event. Understanding this phenomenon is key to accurately interpreting your email performance metrics and maintaining a healthy sender reputation.
The discrepancy typically stems from the technical nuances of email delivery and the automated processes employed by mailbox providers to protect their users. It's not a bug in your reporting, but rather a reflection of how emails are processed and scanned before a final delivery decision is made or even if a human recipient ever sees the message. Let's explore the underlying mechanisms that cause this unusual but common reporting scenario.
How email open tracking works
Email open tracking fundamentally relies on a tiny, transparent image, often called a tracking pixel, embedded within the email's HTML code. When a recipient opens the email and their email client loads images, this pixel is fetched from your server, which then registers an "open" event. This method has long been the standard for measuring email engagement, but it has inherent limitations, especially when automated systems interact with your emails.
The critical point to understand is that not all "opens" are generated by a human reader. Many are the result of automated systems, such as security scanners, antivirus software, or email proxy services. These systems often pre-fetch email content, including tracking pixels, to analyze messages for threats like malware or phishing attempts before they even reach the intended inbox. This automated pre-loading can trigger an open event on your end, even if the email is later deemed undeliverable or the recipient mailbox does not exist.
A notable example of this behavior is Apple Mail Privacy Protection (MPP), which pre-loads all images in an email, regardless of whether the user actually opens it. Similarly, Google's image proxy system also caches images, which can lead to artificial opens being registered. This makes traditional open rate metrics less reliable as a true indicator of human engagement.
Mailbox provider (MBP) security scans
The core of the "open-then-bounce" paradox lies in the SMTP (Simple Mail Transfer Protocol) transaction. When you send an email, the sending server communicates with the receiving mailbox provider's (MBP) server. This communication involves several stages: the initial connection, identifying the sender (MAIL FROM), identifying the recipient (RCPT TO), and finally, transferring the actual email content (DATA).
Some MBPs, particularly those with advanced security protocols, will accept the email content (the DATA command) even if they haven't yet verified the recipient's existence. They do this to perform an in-depth scan of the email's content, links, and attachments for threats. During this scanning process, the tracking pixel is often loaded, registering an "open." Only *after* the scan is complete and the content is deemed safe, does the MBP attempt to deliver the email to the recipient's specific mailbox.
If, at that final stage, the mailbox is found to be non-existent or inactive, the MBP will then generate a bounce message, indicating an "unknown mailbox" or a similar error. This explains why you might see both an open event and a bounce for the same email. It's a timestamp issue: the "open" occurred during the security scan, which happened before the final delivery (or non-delivery) decision was made.
Typical SMTP flow (recipient verified before data)
Initial Check: Receiving server verifies recipient existence at the RCPT TO stage.
Data Transfer: Only if recipient is valid, content (DATA) is sent. Open tracking pixel may load here.
Result: If recipient unknown, bounce occurs early (before DATA). No open registered.
Understanding "unknown mailbox" bounces
An "unknown mailbox" error, typically indicated by a 550 5.1.1 SMTP response code, means the email address you tried to send to does not exist on the receiving server. This is a hard bounce, signaling a permanent delivery failure. When this bounce occurs after an "open" is recorded, it means the receiving server accepted the email's data for scanning before realizing the recipient was invalid.
This can happen for several reasons. Sometimes, an email address might have existed previously but was subsequently deleted or deactivated. In other cases, it could be a simple typo in the email address. Regardless of the cause, the "open" event signals that the email content was processed, even if it ultimately had nowhere to go. This scenario is particularly common when dealing with older lists or domains with aggressive security measures. If you are seeing bounces for a specific domain like AOL, it might indicate changes on their end.
It's crucial to differentiate these automated opens from genuine human engagement. While your reporting system might show an open, it doesn't mean a person interacted with your email. This can significantly skew your metrics and lead to misinformed campaign optimizations. For example, if you see emails hard bouncing after showing opens, it suggests this automated scanning behavior.
These artificial opens have a significant impact on your deliverability metrics. Inflated open rates can give a false sense of success, masking underlying issues like a decaying email list or poor engagement from actual recipients. If your reports show high opens but low clicks or conversions, it's a strong indicator that automated systems are heavily influencing your open data.
Furthermore, continued sending to unknown or invalid mailboxes can negatively affect your sender reputation. While a single bounce won't immediately put you on a blacklist or blocklist, a pattern of sending to non-existent addresses signals poor list hygiene. Mailbox providers interpret this as a sign of a sender who isn't maintaining their list, which can lead to more of your emails being sent to spam folders or being blocked entirely.
Monitoring your bounces closely and promptly removing invalid addresses from your lists is essential. This not only improves your deliverability rates but also ensures that your engagement metrics are more accurate, allowing you to make better strategic decisions. You should also consider how to identify artificial email opens and clicks to get a clearer picture of true engagement.
Traditional open metrics
Definition: Measures when a tracking pixel loads.
Assumption: User has viewed the email content.
Pitfall: Can be inflated by automated scans and privacy features.
True engagement metrics
Focus: Clicks, conversions, replies, and forwards.
Reliability: Stronger indicators of human interaction.
Strategy: Segment based on actual interaction, not just open rate.
A deeper understanding of email reporting
The occurrence of an email open reported alongside an unknown mailbox status is a clear sign that automated systems are at play, pre-scanning your emails before a final delivery decision. While this can inflate your open rates, it highlights the importance of focusing on more reliable engagement metrics like clicks and conversions. Maintaining a clean email list and understanding the nuances of how mailbox providers process emails are crucial for accurate reporting and optimal deliverability.
By recognizing these behind-the-scenes interactions, you can better interpret your email campaign data, refine your strategies, and ensure your messages truly reach and resonate with your intended audience. This deeper understanding will help you navigate the complexities of email deliverability more effectively and build stronger sender reputation over time.
Views from the trenches
Best practices
Actively remove bounced addresses to maintain list hygiene and improve sender reputation.
Segment recipients based on actual clicks and conversions, not just open rates, for more accurate engagement.
Implement DMARC to improve email authentication and gain insight into delivery issues.
Common pitfalls
Solely relying on open rates as a measure of campaign success without accounting for automated scans.
Ignoring bounce reports, leading to sending emails to non-existent or problematic addresses.
Not understanding the difference between SMTP's RCPT TO and DATA stages for delivery decisions.
Expert tips
Use a real-time email verification service to clean your lists before sending campaigns.
Monitor your domain's reputation with tools like Google Postmaster Tools for suspicious activity.
Analyze timestamps of opens and bounces to identify patterns of automated pre-scanning.
Expert view
Expert from Email Geeks says that automated security scanning by spam filters can generate clicks and opens before an email is bounced or delivered.
2021-04-14 - Email Geeks
Marketer view
Marketer from Email Geeks says that tracking pixels measure how machines, not necessarily people, interact with content, so an email can be "opened" and "not exist" simultaneously.
Reliability: Stronger indicators of human interaction.
