Why do all-image emails have higher open rates and are open rates a reliable KPI?

Key findings

• Image loading: Open rates are traditionally measured by the loading of a tiny, invisible tracking pixel (an image). If an email is entirely image-based, recipients are often compelled to enable image loading to view any content, thus artificially inflating the reported open rate.
• ISP pre-fetching: Many Internet Service Providers (ISPs), such as Verizon, pre-fetch emails and their images to scan them for malicious content or to prepare for faster display. This automatic image loading also registers as an open, regardless of whether the user actually viewed the email, further distorting the metric. You can read more about how ISPs track engagement.
• User experience: Forcing image loading can lead to a poor user experience, as many users (particularly on mobile data or with privacy concerns) prefer to defer image downloads.
• Misleading KPI: While historically important, the open rate has become a less reliable Key Performance Indicator (KPI) due to these technical nuances. It may indicate curiosity or effective subject lines, but not necessarily genuine engagement or conversion. This sentiment is echoed by experts who point out the inaccuracy of current open rate measurements.

Key considerations

• Focus on deeper metrics: Prioritize metrics like click-through rate (CTR), click-to-open rate (CTOR), and conversions over raw open rates to gauge true campaign effectiveness.
• Balanced content: Design emails with a balance of text and images, ensuring the core message is readable even if images are blocked by default. Include alt text for all images. Learn how to protect deliverability for image-only emails.
• A/B testing nuances: When conducting A/B tests, ensure proper randomization and statistical significance in your cohorts to avoid drawing false conclusions based on skewed data or temporary variances.
• Deliverability impact: While all-image emails might show higher opens, they can also increase email size, potentially impacting deliverability and placement in the inbox. Consider the long-term impact on sender reputation if engagement metrics are artificially inflated.

Key opinions

• Open rates for subject line testing: Many marketers still use open rates as a primary metric for assessing the effectiveness of subject lines and preview text, viewing it as an indicator of initial curiosity or interest. However, as Mailchimp suggests, this should be part of a broader analysis including CTOR. You can find out more about CTOR versus open rate.
• Focus on conversions: The ultimate goal of most email campaigns is not just to be opened, but to drive conversions (purchases, sign-ups, etc.). If high open rates don't translate into higher conversions, their value as a KPI is diminished. Even if Gmail open rates are low, focusing on conversions is key.
• Perception vs. reality: Some marketers acknowledge that while open rates are often what boards of companies crave, they may not accurately reflect true engagement or campaign success.
• Testing methodology matters: It's crucial to ensure A/B testing groups are homogeneous and statistically randomized to avoid drawing erroneous assumptions from observed results.

Key considerations

• Holistic metric analysis: Combine open rates with other metrics like click-through rate (CTR), conversions, and revenue per email to get a more complete picture of campaign performance. Understanding why your deliverability rate is wrong is important.
• Transparency in design: Avoid designs that require images to be loaded for the main content to be visible, as this can frustrate users and lead to artificially inflated open rates without genuine engagement.
• Adapting to industry changes: As ISPs evolve their practices (e.g., Apple Mail Privacy Protection), traditional open rate tracking becomes even less accurate. Marketers must adapt their measurement strategies. Salesforce suggests that email open rates can still be useful if looked at in aggregate.
• User experience focus: Prioritize creating a positive user experience. Emails that are easy to read and digest, regardless of image loading, are more likely to foster genuine engagement and long-term subscriber loyalty.

Key opinions

• Open rates are not KPIs: Experts strongly argue that open rates have never been a true Key Performance Indicator (KPI) because they do not directly measure the performance of a business objective (e.g., sales, sign-ups). They are a metric, useful for diagnosing issues, but not for optimizing overall performance. You can read more about good email engagement thresholds for deliverability.
• Artificial inflation: All-image emails can artificially inflate open rates by forcing recipients to load images to see any content, thereby triggering the tracking pixel. This is considered a user-hostile practice that games the metric rather than reflecting genuine interest.
• Impact of pre-fetching: Mail providers (e.g., Verizon) pre-fetch emails, generating fake opens that distort open rate data. Experts predict a future where marketers must move away from open rates as a primary KPI due to such practices.
• Diagnostic tool vs. optimization target: Open rates can be useful for diagnosing deliverability issues or testing subject lines, but they should not be the ultimate goal for optimization. Over-optimizing for open rates can lead to strategies that don't benefit overall campaign performance.

Key considerations

• Focus on "real" KPIs: Marketers should shift their focus to metrics that directly reflect business goals, such as click-through rates, conversion rates, and revenue. These provide a more accurate picture of an email's true value.
• A/B test rigor: For A/B testing, it's vital to ensure that test groups are homogeneous and statistically randomized. Ignoring cohort variances can lead to erroneous assumptions about campaign outcomes, especially when interpreting metrics like open rates. This highlights the importance of rigorous accurate measurement.
• ISP-specific analysis: When higher open rates are observed, examine individual ISP open rates. If a particular ISP shows disproportionately high opens, it could be due to caching or pre-fetching behavior, especially for all-image emails, rather than genuine user interaction.
• Avoid gaming metrics: Building emails to force image loads just to inflate open rates can be detrimental in the long run. Such practices can lead to poor sender reputation if ISPs detect artificial engagement or if recipients mark emails as spam due to a bad user experience.

Key findings

• Pixel-based tracking: Open rates are traditionally tracked by embedding a 1x1 pixel image within the email. When this image is downloaded by the recipient's email client, it registers an open. This is the fundamental mechanism. This mechanism also underlies why hidden links can get high click rates.
• Image blocking defaults: Many email clients and webmail interfaces block images by default for security and privacy reasons. An open is only recorded if the user explicitly enables image loading or if the client automatically downloads them.
• Privacy features: Modern privacy features (e.g., Apple Mail Privacy Protection) pre-load all email content, including tracking pixels, before delivering it to the user. This makes it impossible to distinguish between a genuine human open and an automated pre-fetch, rendering traditional open rates unreliable. This affects Yahoo and AOL open rates as well.
• Engagement signals: ISPs and mail providers use a variety of engagement signals beyond just opens (e.g., clicks, replies, forwards, moving to folders, marking as not spam) to assess sender reputation and inbox placement. Open rates alone are insufficient.

Key considerations

• Redefined metrics: Email platforms and analytics providers are adapting to these changes by focusing on more reliable metrics (e.g., unique clicks, conversions) or by adjusting how they report open rates to account for pre-fetching.
• Content accessibility: Documentation often recommends designing emails that are accessible and readable even with images turned off (e.g., using alt text, balancing text-to-image ratio). This ensures the message is conveyed regardless of tracking pixel behavior. Campaign Monitor provides a knowledge base article on what constitutes good email metrics.
• Beyond the open: The industry is increasingly moving towards a model where engagement is measured by actual interaction with links and content rather than just whether an email appears to have been opened.
• Consider deliverability impact: While all-image emails might trick open rate metrics, they can be larger in file size, which some documentation suggests might indirectly affect deliverability or load times for recipients with slower connections, potentially leading to a poorer overall experience.