Email open rate

Email open rate measures the percentage of delivered emails that recipients open. It is tracked by embedding a tiny invisible image (a tracking pixel) in the email; when the recipient's email client loads this image, the email is counted as opened.

Open rate has been a foundational email marketing metric for decades because it indicates whether your subject lines, sender name, and send timing are effective at getting recipients to engage with your messages. A strong open rate suggests your audience recognises your brand, finds your subject lines compelling, and checks their email during your send window.

However, email open rate has become significantly less reliable since Apple introduced Mail Privacy Protection (MPP) in iOS 15. MPP pre-loads all email content, including tracking pixels, regardless of whether the user actually reads the email. This inflates open rates for Apple Mail users, which represents a significant share of consumer email opens. Similar privacy features are being adopted by other email clients.

Despite these limitations, open rate remains useful as a relative metric. While absolute open rates may be inflated, comparing open rates between campaigns, subject lines, or segments still reveals which approaches perform better, as long as the same privacy inflation applies consistently across the comparison.

Open rate is calculated by dividing the number of opened emails by the number of delivered emails and multiplying by 100. Delivered emails exclude bounces, both hard bounces (invalid addresses) and soft bounces (full inboxes or temporary server issues).

The tracking mechanism relies on a 1x1 pixel image embedded in the email HTML. When the email client loads this image from the tracking server, the open is recorded. This means that plain-text emails cannot track opens, emails viewed with images disabled are not counted as opened, and privacy features that pre-load images will register false opens.

Most email platforms report both total opens and unique opens. Total opens count every time the email is opened, including repeat views. Unique opens count only one open per recipient. Unique open rate is the more useful metric because it tells you what percentage of your audience engaged, rather than how many times engaged recipients re-read the message.

Open rate sits early in the email marketing funnel, determining how many recipients engage with your message before the click and conversion stages.

The tree shows that open rate is driven by subject line quality, sender reputation, send timing, and list segmentation. Improving any of these inputs increases the number of recipients who see your email content, which increases the potential for clicks and conversions downstream.

The tree also reveals a critical dependency: deliverability rate sits upstream of open rate. If emails are not reaching inboxes in the first place, even the best subject line cannot generate opens. Maintaining sender reputation through list hygiene and engagement-based sending is a prerequisite for strong open rates.

1. 1

   Write compelling subject lines

   The subject line is the primary driver of open rate. Keep it under 50 characters. Use specificity over vagueness, numbers and data points, curiosity gaps, and personalisation. A/B test subject lines on a portion of your list before sending to the full audience.

2. 2

   Optimise send time and frequency

   Send when your audience is most likely to check email. B2B audiences tend to engage mid-morning on weekdays. B2C audiences vary by segment. Use send-time optimisation features to deliver to each recipient at their peak engagement time. Tracking bounce rate alongside open rate helps distinguish delivery issues from engagement issues.

3. 3

   Segment your list and personalise

   Segmented campaigns outperform mass emails by 2x or more on open rate. Segment by behaviour, engagement level, purchase history, or lifecycle stage. Personalised subject lines that reference the recipient's actions or interests drive higher open rates.

4. 4

   Maintain list hygiene

   Remove inactive subscribers, fix invalid addresses, and re-engage or suppress unresponsive contacts. A smaller, engaged list will have a higher open rate and better deliverability than a large, stale list.

5. 5

   Build sender reputation

   Authenticate your domain with SPF, DKIM, and DMARC. Use a consistent sender name and address. Avoid spam trigger words in subject lines. High deliverability is the prerequisite for high open rates.