Definition
Unique opens counts the number of distinct recipients who opened an email at least once, rather than counting every individual open event. If a recipient opens the same email three times, it counts as one unique open but three total opens. The unique open rate is calculated by dividing unique opens by emails delivered.
Unique opens provide a more accurate picture of campaign reach. Total opens can be inflated by a small number of recipients who open repeatedly. However, Apple's Mail Privacy Protection (MPP), introduced in 2021, has made unique open measurement unreliable for Apple Mail users, who represent 55–60% of email opens — their email client pre-loads content and registers a "unique open" whether they read the email or not.
Unique Opens vs Total Opens
| Metric | What It Measures | Best For |
|---|---|---|
| Unique opens | Distinct recipients who opened | Understanding campaign reach |
| Total opens | All open events including repeats | Understanding engagement depth |
| Open rate (unique) | Unique opens / delivered | Standard industry benchmark |
Best Practices
- Use unique open rate as your standard open-rate metric
- Recognise that MPP has made open-rate comparisons unreliable over time
- Compare unique open rates within the same campaign to spot trends
- Use click rate and conversion rate as primary metrics, with open rate as directional
- Segment open analysis by likely MPP users vs non-MPP users if your ESP supports it
- Avoid using unique opens as a engagement signal for list-hygiene decisions
Related Glossary Terms
Abandoned Cart Email
An abandoned cart email is an automated message sent to customers who added items to their online shopping cart but left without completing the purchase. It is one of the highest-converting email types in ecommerce.
ARPU (Average Revenue Per User)
ARPU (Average Revenue Per User) is a metric that measures the average revenue generated per email subscriber over a specific period, used to evaluate list value and campaign effectiveness.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.
Click-Through Rate
Click-through rate (CTR) is the percentage of email recipients who clicked one or more links in your email campaign. It measures how compelling your content and call-to-action are.
Click-to-Convert Rate
Click-to-convert rate measures the percentage of email clicks that result in a desired conversion action such as a purchase, signup, or download. It shows how effective your post-click experience is at turning interest into results.
Click-to-Open Rate
Click-to-open rate (CTOR) is the percentage of email opens that resulted in at least one click. It measures how compelling your email content is for people who already opened it.
Frequently Asked Questions
Unique opens are still useful as a directional trend metric — a campaign with uniquely low unique open rate likely has subject-line or deliverability issues. However, they should no longer be treated as a precise measurement due to MPP inflation. Unique opens are most valuable when compared within the same audience over time rather than taken as absolute numbers.
Unique open rate = (Number of unique recipients who opened / Number of emails delivered) × 100. Most ESPs calculate this automatically. The "unique" count de-duplicates multiple opens from the same recipient, so each person only contributes one open to the numerator.