# Open & Unique Open

*Analytics*. The record of each time a customer opens your Twilio Email email messages.

## Open tracking in Twilio Email

When turned on, Twilio Email inserts a small, transparent image into all emails. When a customer reads an email, their [Mail User Agent (MUA)][mua] loads the tracking image. Twilio Email then registers the open event.

The *opens percentage* divides the total number of times your users opened your emails by the total number of Delivered messages.

The *unique opens percentage* divides the number of unique individuals that have opened your emails by the total number of Delivered messages.

Twilio stores unique open events for up to seven days.

[mua]: /docs/glossary/mua