When youcall`track` from one of Segment's mobile or server-side libraries, Segment keys the user with the`userId` and also provides the Klaviyo`$email``customer_property` if your`userId` is an email, or you provide`email` as one of your event`properties`.

When youmake a Track call from one of Segment's mobile or server-side libraries, Segment keys the user with the`userId` and also provides the Klaviyo`$email``customer_property` if your`userId` is an email, or you provide`email` as one of your event`properties`.

Segment also maps the following Segment spec'd properties to Klaviyo's[special people properties](https://developers.klaviyo.com/en/docs/javascript_api#identify-people){:target="_blank"}:.