Do ISPs re-fetch email tracking images, and what causes delayed email opens?

Key findings

• Image caching: ISPs and email clients (like Gmail's image proxy) cache images, including tracking pixels, to improve loading times and user privacy. Once cached, the image might not be refetched unless the cache expires or the email is opened on a different client.
• Refetching scenarios: Images may be refetched if a user accesses the email on a new device or client, or if the initial cache expires, leading to a new open event being registered for an old email. This can explain some inaccurate open tracking issues.
• Delayed opens: While most opens occur shortly after receipt, some legitimate opens can happen weeks or even months later if the recipient revisits the email. This is particularly true for reference or transactional emails.
• Bot activity: Although less common for delayed opens, bots or security scanners can pre-fetch images upon receipt, which might falsely inflate initial open rates. It would be unusual for bots to trigger opens long after an email has been received.

Key considerations

• Open rate accuracy: Open rates are not always perfectly accurate indicators of engagement due to caching, proxy services, and bot activity. For more on this, see how accurate email open rates are.
• Engagement metrics: Focus on other engagement metrics like clicks, conversions, and replies for a more comprehensive view of subscriber interest, especially if you observe a high volume of old opens. Low open rates might indicate deliverability issues.
• Auditing old opens: Analyze the pattern of delayed opens. If it's a small, consistent percentage, it could be legitimate behavior. Unusual spikes might warrant further investigation into the source of the traffic.
• List hygiene: Regularly clean your email list to remove inactive subscribers, reducing the likelihood of unusual open activity from old, potentially abandoned, email addresses. This helps maintain a good sender reputation.

Key opinions

• Single fetch assumption: Many marketers assume ISPs only fetch tracking images once upon initial email rendering or pre-fetching.
• Unexpected opens: A noticeable number of 'opens' occurring days or weeks after an email was sent raises questions about image refetching or bot activity, leading to concerns about unintentional opens and clicks.
• Legitimate delays: Some marketers acknowledge that legitimate delayed opens occur, especially for content that remains relevant over time.
• Bot concerns: Bots are a recurring concern, potentially skewing open rate data. Marketers are looking for ways to diagnose an unexpected drop in engagement.

Key considerations

• Understanding proxies: Marketers need to understand how Gmail's image proxy affects open tracking and other ISP caching mechanisms to interpret open rates correctly.
• Segment analysis: Conducting detailed segment analysis can help identify patterns in open behavior, such as a segment showing more delayed opens.
• Defining engagement: Develop a clearer definition of 'active engagement' that accounts for the nuances of tracking, possibly relying more on clicks than opens for critical segments.
• Deliverability impact: Consider whether delayed email opening negatively impacts deliverability, as inconsistent engagement patterns can sometimes be flagged by ISPs.

Key opinions

• Caching and expiry: Experts confirm that pre-fetched images are cached by ISPs and email clients, and these cached images will eventually expire.
• Client-side refetching: If an email client (e.g., Mail.app on macOS) pre-fetches an image, and the user subsequently opens the same email on a different client (like a mobile device), the image will be refetched, registering a new 'open'.
• Bot behavior: Machine-generated image loads from bots are usually associated with the time of email receipt, not significantly later, making them unlikely causes for very delayed opens.
• Recipient action: If an image is fetched long after initial receipt, it is highly probable that a direct action by the recipient (e.g., opening the email) triggered it.

Key considerations

• Understanding proxies: ISPs like Google use image proxies that affect open tracking, potentially reporting opens when the email is just downloaded, not necessarily viewed.
• Engagement patterns: Analyzing the distribution of opens over time can reveal patterns of genuine delayed engagement versus anomalies.
• Multi-client opens: Be aware that users checking email on multiple devices can legitimately trigger multiple or delayed open events for the same email.
• Deliverability impact: While delayed opens are generally less impactful than bounces or spam complaints, consistent very delayed opens might indicate issues with recipient list hygiene, which can affect overall how ISPs track email engagement.

Key findings

• Image proxy behavior: Major ISPs like Gmail proxy images, which means their servers fetch the image, not the user's client directly. This fetch often occurs upon receipt or pre-rendering, not necessarily when the user views the email.
• Caching policies: Proxies cache images, and the duration of this cache depends on server configurations and image headers. Expired caches necessitate a refetch upon subsequent email access.
• Multiple device access: When a user accesses an email from different devices or email clients, each client may fetch the images independently if not previously cached or if its cache has expired, leading to multiple open registrations.
• Security scans: Some corporate or personal security software might scan emails upon arrival, triggering image loads even before a user opens the email, which can lead to early or unintended open registrations.

Key considerations

• Tracking pixel design: The design of tracking pixels (e.g., unique URLs per recipient) is essential for accurate attribution of opens.
• SMTP transaction logs: For understanding delays, reviewing SMTP transaction logs can provide insights into when the receiving server actually accepted the email, which might differ significantly from the sent time.
• Email deliverability issues: High deferral rates or email delivery failures can cause legitimate delays in opens, indicating a need to investigate sender reputation or infrastructure.
• RFC compliance: Adhering to email RFCs and best practices can minimize technical reasons for delivery delays and ensure more reliable open tracking, though ISP behavior can still vary. See what RFC 5322 says versus what works.