> For the complete documentation index, see [llms.txt](https://docs.mediarithmics.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.mediarithmics.io/setup-and-monitor/collect-and-manage-your-data/design-your-data-model-and-schema/user-identity/user-identifiers/user-email.md).

# User Email

Identify a user by their email address, using a hash consistently across every data flow.

## Your goal

Use the email address as an identifier without moving the address around more than you need to - and hash it the same way everywhere, so the same person matches across your flows.

## Expected outcome

One hash function agreed for the datamart and used by every integration that sends or receives email-based identifiers.

## What a UserEmail is

A **UserEmail** is an identifier based on the user's email address. It is usually derived from the address by a hash function such as MD5 or SHA-256. The email can be hashed directly by the platform.

A user can have several UserEmails registered.

| Property        | Type                | Description                                    |
| --------------- | ------------------- | ---------------------------------------------- |
| `hash`          | ID                  | The hashed email.                              |
| `email`         | String, optional    | The user's email, not hashed.                  |
| `creation_ts`   | Timestamp           | When the email was registered on the platform. |
| `expiration_ts` | Timestamp, optional | The email's eventual expiration timestamp.     |

The `hash` property is **mandatory** and is the property used to identify a user. It is provided as `$hash` in user activity requests.

{% hint style="warning" %}
**Always use the same hashing function** in your datamart and in all its integrations. Matching between data flows depends on it: a SHA-256 hash and an MD5 hash of the same address are two different identifiers, and neither will match the other.
{% endhint %}

## In the schema

```graphql
type UserEmail {
  id: ID! @TreeIndex(index:"USER_INDEX")
  creation_ts: Timestamp! @TreeIndex(index:"USER_INDEX")
  last_activity_ts: Timestamp @TreeIndex(index:"USER_INDEX")

  email: String @TreeIndex(index:"USER_INDEX")
}
```

## In an activity

```json
{
  "$user_identifiers": [
    {
      "$type": "USER_EMAIL",
      "$hash": "<the email hash>",
      "$email": "<the raw email, optional>"
    }
  ]
}
```

## Where the hash is used elsewhere

The email hash is one of the identifiers accepted by the endpoints reachable with [signature authentication](/setup-and-monitor/manage-the-platform/manage-api-access.md#signature-authentication), which is how a public application can look up a user's segments without carrying a long-term token.

It is also one of the technical names available for [segment metrics](/setup-and-monitor/plan-and-integrate/set-up-the-environment/set-up-segment-metrics.md), which is how teams see how many users in a segment are reachable by email.

## Verify

* Every integration sending email-based identifiers uses the agreed hash function.
* A test user identified by email hash resolves to the expected UserPoint.
* Segment metrics counting emails return plausible values.

## Related pages

<table data-view="cards" data-search="false"><thead><tr><th></th><th></th><th data-hidden data-card-target data-type="content-ref"></th></tr></thead><tbody><tr><td><strong>User Account</strong></td><td>The other person-level identifier.</td><td><a href="/setup-and-monitor/collect-and-manage-your-data/design-your-data-model-and-schema/user-identity/user-identifiers/user-account.md">User Account</a></td></tr><tr><td><strong>Email tracking</strong></td><td>Collecting email views and clicks.</td><td><a href="/setup-and-monitor/collect-and-manage-your-data/collect-data-in-real-time/email-tracking.md">Email tracking</a></td></tr><tr><td><strong>Import identifiers</strong></td><td>Sending email identifiers in batch.</td><td><a href="/setup-and-monitor/collect-and-manage-your-data/import-data-in-batch/import-identifiers.md">Import identifiers</a></td></tr></tbody></table>


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