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

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.

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.

Email marketer from Gmass says, email privacy protection affects open rates, and marketers may need to measure conversions from clicks to measure email marketing success.

Email marketer from Email Geeks shares an example of Google 'opening' a test email before delivery and actual user interaction. This illustrates how bot impressions of tracking pixels can skew open rates if not accounted for.

Email marketer from Email Geeks explains that an unknown user response may come after data and not after rcpt, allowing the recipient server to scan content and trigger an open if the tracking pixel is loaded remotely. Some blocklists do this, potentially causing DOI challenges to be 'clicked'.

Email marketer from Email Geeks stresses that tracking pixels measure machine interactions, not necessarily human ones. Therefore, an email can be 'opened' by a machine scanning it en route, even if the mailbox doesn't exist. Meaning the mail was opened and the mailbox doesn’t exist can be 'true' at the same time.

Email marketer from EmailVendorForum.com highlights that many email providers now use pre-fetch mechanisms. These mechanisms proactively download images in emails for security or rendering purposes. This causes the open pixel to fire, but the email may have been deleted before delivery and never seen by the user.

Email marketer from Reddit explains that in a corporate environment, emails often pass through aggressive spam filters that scan all content, including images. These scans can register an open before the email even reaches the intended (but possibly invalid) mailbox.

Email marketer from Sendinblue answers that the email showing as opened and unknown mailbox could be because some security software scans emails and clicks links/images as part of their process, thus triggering a false 'open'.

Email marketer from Email Geeks explains that the classifications are bounce categories applied by the ESP, and the recipient domain and exact bounce message would offer more information. Also explains that the software may consider a link click (due to malware scanning) as an 'open', hence the 'date opened' timestamp, followed by the actual bounce with its timestamp for 'date bounced'.

Email marketer from EmailGeeks shares that, anti-spam policies and algorithms are constantly evolving. Open-tracking has become an unreliable metric for user behavior as the receiving mail server may be opening it. The actual opens by customers who are interested in your email will be much less.

Email marketer from Neil Patel Digital explains that an email showing as 'opened' despite an unknown mailbox can be due to bots or security scanners triggering the tracking pixel. These automated systems can access the email content and load images, leading to a false open signal even if the email never reaches a real recipient.

Email marketer from StackOverflow explains that the bounce is asynchronous from the open tracking request (if the email client displays the pixel). The 'open' is recorded as soon as the pixel is loaded and requested, but the bounce may happen after the receiving mail server processes and validates the email addresses. Some bounces happen after a delay (e.g. spam trap)

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.

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.

Expert from Spamresource explains that it can be a bot, AV scanner, or preview pane opening the message. A receiving mail server can see the request to display an image and count the message as read, even if the mailbox is invalid.

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.

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.

Documentation from Microsoft explains that Exchange servers generate Non-Delivery Reports (NDRs) for failed deliveries. However, even if an NDR is eventually sent (indicating an invalid mailbox), the initial processing of the email *might* still involve scanning for malware/spam, which could trigger an open if tracking pixels are present.

Documentation from AWS explains that bounces indicate a permanent or transient issue with delivery. While bounces indicate delivery failures, a separate process might still involve scanning the email contents (including tracking pixels) *before* the final bounce determination is made.

Documentation from SparkPost answers that it's possible to see an open event before a bounce if the recipient server's security system scans the email and loads images before the email address is validated. This often occurs with spam filters or automated security checks.