diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6bd3a7e..38816ee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,19 @@
 # Changelog
 
+## Unreleased
+
+- `AppSubscription::checkout()` takes `billElapsedPeriods` and `maxNetAmount`: a subscription
+  started more than one period ago is billed for its elapsed periods too, up to the given net
+  amount (409 `elapsed_amount_exceeded` with `net_amount`, `elapsed_periods_not_allowed`). Calls
+  without the new arguments send the same request as before.
+- `ApiException::body()` returns the JSON body of Anikeen ID's error response, e.g. its
+  `net_amount`.
+- Add `AnikeenId::findUserByEmail()` for trusted apps: looks up an account by email address
+  (regardless of case) without creating one, null when there is none.
+- `createUser()` documents `name`, `password_confirmation` and `send_invitation` (a newly created
+  account gets an invitation mail to set a password) and the response fields `created` and
+  `invitation_sent`.
+
 ## 3.1.0 (2026-11-05)
 
 - Add the scope `products:read` (`Scope::PRODUCTS_READ`) for read-only access to the product APIs
diff --git a/README.md b/README.md
index 1da3dae..af76b77 100644
--- a/README.md
+++ b/README.md
@@ -514,6 +514,11 @@ $subscription = $subscriptions->create([
 
 $subscription->checkout();             // bill the first period
 $subscription->checkout(startsAt: $domain->registered_at); // already running: bill the period that started then
+$subscription->checkout(                // running for longer: also bill the periods that have ended
+    startsAt: $domain->registered_at,
+    billElapsedPeriods: true,
+    maxNetAmount: '15.57',             // net ceiling, e.g. the amount the customer agreed to
+);
 $subscription->pause();                // expires when the paid period ends
 $subscription->resume();               // expired subscriptions start a new period
 $subscription->revoke(refund: false);  // cancel
@@ -526,10 +531,44 @@ The user's Anikeen ID is read from the billable's `anikeen_id` attribute
 (`AnikeenId::useUserIdField('…')` to change it). The app must be trusted by Anikeen ID; it sees the
 subscriptions it created (plus those of clients delegated to it).
 
+#### Starting from an earlier date
+
+`checkout(startsAt: …)` books a service that is already running, e.g. a domain registered before
+the customer came to your app. If the first period from that date is still running, it is billed
+now and renews when it ends. An older start is refused with `period_already_ended`, unless you
+pass `billElapsedPeriods: true` with `maxNetAmount` (net, up to two decimals; both together, or
+`checkout()` throws an `InvalidArgumentException` without sending anything):
+
+- every period that has ended plus the running one is billed now, as one line with the full
+  service period. It lands on the user's collective order of the current month and is invoiced and
+  charged with the next monthly bill; nothing is back-dated.
+- the start may be at most about three years ago (422 otherwise).
+- if the periods come to more than `maxNetAmount`, nothing is booked: 409
+  `elapsed_amount_exceeded`, with the real amount in `$e->body()->net_amount` (e.g. `"20.76"`), so
+  you can show it and retry.
+- 409 `elapsed_periods_not_allowed` when the subscription can't be billed this way (e.g. it ran
+  before, replaces another one, or is billed by the hour or day with a monthly cap).
+
+```php
+try {
+    $subscription->checkout(startsAt: $startsAt, billElapsedPeriods: true, maxNetAmount: $confirmed);
+} catch (ApiException $e) {
+    if ($e->error === 'elapsed_amount_exceeded') {
+        $actual = $e->body()->net_amount; // ask again with the real amount
+    } else {
+        throw $e;
+    }
+}
+```
+
+#### Error codes
+
 Failures throw `Anikeen\Id\Exceptions\ApiException` with `status` (0 = not reachable), a stable
 `error` code (`already_active`, `already_cancelled`, `not_active`, `already_paused`,
 `already_resumed`, `billing_address_missing`, `not_changeable`, `subscription_not_found`,
-`idempotency_key_conflict`) and `isNotFound()`, `isConflict()`, `isTemporary()`:
+`idempotency_key_conflict`, `period_already_ended`, `elapsed_periods_not_allowed`,
+`elapsed_amount_exceeded`), the error response via `body()` and `isNotFound()`, `isConflict()`,
+`isTemporary()`:
 
 ```php
 try {
@@ -616,6 +655,29 @@ While a session is open, its subscriptions can't be checked out or repriced by t
 `subscription.activated` / `subscription.cancelled` are still sent per subscription, before the
 `checkout.*` event of the session.
 
+### User accounts (trusted apps, no user token)
+
+Trusted apps can look up and create Anikeen ID accounts with the app's own token. Email addresses
+are matched regardless of case.
+
+```php
+$anikeenId = app(AnikeenId::class);
+
+$account = $anikeenId->findUserByEmail('erika@example.com'); // null if no account uses it
+$account?->data->id;                   // also name, email, email_verified, has_billing_address
+
+$result = $anikeenId->createUser([     // finds the account of the email address or creates it
+    'name' => 'Erika Mustermann',
+    'email' => 'erika@example.com',
+    'send_invitation' => true,         // a new account gets a mail with a link to set a password
+]);
+$result->data->created;                // false: the account already existed (and is unchanged)
+$result->data->invitation_sent;        // only for an account created by this call without a password
+```
+
+`findUserByEmail()` changes nothing. It throws `ApiException` when the request fails for any reason
+other than an unknown address.
+
 
 ## API authentication
 
@@ -833,6 +895,7 @@ public function sshKeysByUserId(string $userId): SshKeys
 ```php
 public function getAuthedUser(): Result
 public function createUser(array $attributes): Result
+public function findUserByEmail(string $email): ?Result
 public function findOrCreateSocialiteUser(string $provider, string $providerId, ?string $email = null, ?string $name = null): Result
 public function refreshToken(string $storedRefreshToken, string $scope = ''): Result
 public function isEmailExisting(string $email): Result
diff --git a/README.stub b/README.stub
index 6fb0a71..e0efbcf 100644
--- a/README.stub
+++ b/README.stub
@@ -514,6 +514,11 @@ $subscription = $subscriptions->create([
 
 $subscription->checkout();             // bill the first period
 $subscription->checkout(startsAt: $domain->registered_at); // already running: bill the period that started then
+$subscription->checkout(                // running for longer: also bill the periods that have ended
+    startsAt: $domain->registered_at,
+    billElapsedPeriods: true,
+    maxNetAmount: '15.57',             // net ceiling, e.g. the amount the customer agreed to
+);
 $subscription->pause();                // expires when the paid period ends
 $subscription->resume();               // expired subscriptions start a new period
 $subscription->revoke(refund: false);  // cancel
@@ -526,10 +531,44 @@ The user's Anikeen ID is read from the billable's `anikeen_id` attribute
 (`AnikeenId::useUserIdField('…')` to change it). The app must be trusted by Anikeen ID; it sees the
 subscriptions it created (plus those of clients delegated to it).
 
+#### Starting from an earlier date
+
+`checkout(startsAt: …)` books a service that is already running, e.g. a domain registered before
+the customer came to your app. If the first period from that date is still running, it is billed
+now and renews when it ends. An older start is refused with `period_already_ended`, unless you
+pass `billElapsedPeriods: true` with `maxNetAmount` (net, up to two decimals; both together, or
+`checkout()` throws an `InvalidArgumentException` without sending anything):
+
+- every period that has ended plus the running one is billed now, as one line with the full
+  service period. It lands on the user's collective order of the current month and is invoiced and
+  charged with the next monthly bill; nothing is back-dated.
+- the start may be at most about three years ago (422 otherwise).
+- if the periods come to more than `maxNetAmount`, nothing is booked: 409
+  `elapsed_amount_exceeded`, with the real amount in `$e->body()->net_amount` (e.g. `"20.76"`), so
+  you can show it and retry.
+- 409 `elapsed_periods_not_allowed` when the subscription can't be billed this way (e.g. it ran
+  before, replaces another one, or is billed by the hour or day with a monthly cap).
+
+```php
+try {
+    $subscription->checkout(startsAt: $startsAt, billElapsedPeriods: true, maxNetAmount: $confirmed);
+} catch (ApiException $e) {
+    if ($e->error === 'elapsed_amount_exceeded') {
+        $actual = $e->body()->net_amount; // ask again with the real amount
+    } else {
+        throw $e;
+    }
+}
+```
+
+#### Error codes
+
 Failures throw `Anikeen\Id\Exceptions\ApiException` with `status` (0 = not reachable), a stable
 `error` code (`already_active`, `already_cancelled`, `not_active`, `already_paused`,
 `already_resumed`, `billing_address_missing`, `not_changeable`, `subscription_not_found`,
-`idempotency_key_conflict`) and `isNotFound()`, `isConflict()`, `isTemporary()`:
+`idempotency_key_conflict`, `period_already_ended`, `elapsed_periods_not_allowed`,
+`elapsed_amount_exceeded`), the error response via `body()` and `isNotFound()`, `isConflict()`,
+`isTemporary()`:
 
 ```php
 try {
@@ -616,6 +655,29 @@ While a session is open, its subscriptions can't be checked out or repriced by t
 `subscription.activated` / `subscription.cancelled` are still sent per subscription, before the
 `checkout.*` event of the session.
 
+### User accounts (trusted apps, no user token)
+
+Trusted apps can look up and create Anikeen ID accounts with the app's own token. Email addresses
+are matched regardless of case.
+
+```php
+$anikeenId = app(AnikeenId::class);
+
+$account = $anikeenId->findUserByEmail('erika@example.com'); // null if no account uses it
+$account?->data->id;                   // also name, email, email_verified, has_billing_address
+
+$result = $anikeenId->createUser([     // finds the account of the email address or creates it
+    'name' => 'Erika Mustermann',
+    'email' => 'erika@example.com',
+    'send_invitation' => true,         // a new account gets a mail with a link to set a password
+]);
+$result->data->created;                // false: the account already existed (and is unchanged)
+$result->data->invitation_sent;        // only for an account created by this call without a password
+```
+
+`findUserByEmail()` changes nothing. It throws `ApiException` when the request fails for any reason
+other than an unknown address.
+
 
 ## API authentication
 
diff --git a/src/Id/Concerns/ManagesUsers.php b/src/Id/Concerns/ManagesUsers.php
index 68824e5..26b07a7 100644
--- a/src/Id/Concerns/ManagesUsers.php
+++ b/src/Id/Concerns/ManagesUsers.php
@@ -5,7 +5,9 @@ namespace Anikeen\Id\Concerns;
 
 use Anikeen\Id\ApiOperations\Get;
 use Anikeen\Id\ApiOperations\Post;
+use Anikeen\Id\Exceptions\ApiException;
 use Anikeen\Id\Result;
+use Anikeen\Id\Support\AppRequest;
 use Throwable;
 
 trait ManagesUsers
@@ -24,20 +26,32 @@ trait ManagesUsers
     }
 
     /**
-     * Creates a new user on behalf of the current user.
+     * Finds the account of an email address or creates it (trusted apps, app token). The email
+     * address is matched regardless of case; a new account stores it in lower case, unverified.
+     * An existing account is returned unchanged (only a missing name is filled in).
      *
      * @param array{
-     *       first_name: null|string,
-     *       last_name: null|string,
-     *       username: null|string,
+     *       name?: null|string,
+     *       first_name?: null|string,
+     *       last_name?: null|string,
+     *       username?: null|string,
      *       email: string,
-     *       password: null|string
+     *       password?: null|string,
+     *       password_confirmation?: null|string,
+     *       send_invitation?: bool
      *   } $attributes The user data
-     *     - first_name:  The first name (optional)
-     *     - last_name:   The last name (optional)
-     *     - username:    The username (optional)
-     *     - email:       The email (required)
-     *     - password:    The password (optional, can be reset by the user if not provided)
+     *     - name:                  The display name (optional)
+     *     - first_name:            The first name (optional, joined with last_name into name)
+     *     - last_name:             The last name (optional, joined with first_name into name)
+     *     - username:              The username (optional)
+     *     - email:                 The email (required)
+     *     - password:              The password (optional, can be reset by the user if not provided)
+     *     - password_confirmation: Defaults to password
+     *     - send_invitation:       Send the user an invitation mail with a link to set a password
+     *                              (optional, only when this call created the account without a
+     *                              password, never for an existing account)
+     * @return Result `data` is the user plus `created` (this call created the account) and
+     *     `invitation_sent` (the invitation mail was sent); 201 when created, 200 when it existed
      * @throws Throwable
      */
     public function createUser(array $attributes): Result
@@ -57,6 +71,30 @@ trait ManagesUsers
         return $this->asApp()->post('v1/admin/users', $attributes);
     }
 
+    /**
+     * Looks up the account of an email address without creating or changing anything (trusted
+     * apps, app token). The email address is matched regardless of case.
+     *
+     * `data` is `object{id: string, name: ?string, email: string, email_verified: bool,
+     * has_billing_address: bool}`.
+     *
+     * @return Result|null null when no account uses the email address
+     * @throws ApiException when the request fails for another reason
+     */
+    public function findUserByEmail(string $email): ?Result
+    {
+        try {
+            return AppRequest::send('POST', 'v1/admin/users/lookup', ['email' => $email]);
+        } catch (ApiException $e) {
+            // only Anikeen ID's own answer means "no account", not e.g. an unknown route
+            if ($e->isNotFound() && $e->error === 'user_not_found') {
+                return null;
+            }
+
+            throw $e;
+        }
+    }
+
     /**
      * Find the user of a social login or create it (trusted apps, app token).
      *
diff --git a/src/Id/Exceptions/ApiException.php b/src/Id/Exceptions/ApiException.php
index 5654568..237cf16 100644
--- a/src/Id/Exceptions/ApiException.php
+++ b/src/Id/Exceptions/ApiException.php
@@ -3,6 +3,7 @@
 namespace Anikeen\Id\Exceptions;
 
 use Anikeen\Id\Result;
+use Psr\Http\Message\ResponseInterface;
 use Throwable;
 
 /**
@@ -44,6 +45,24 @@ class ApiException extends ResourceException
         return new static(sprintf('Anikeen ID is not reachable: %s', $previous->getMessage()), 0, null, $previous);
     }
 
+    /**
+     * The JSON body of Anikeen ID's error response, null without one. Some errors carry details,
+     * e.g. `net_amount` of "elapsed_amount_exceeded".
+     */
+    public function body(): ?object
+    {
+        $previous = $this->getPrevious();
+        $response = $previous !== null && method_exists($previous, 'getResponse') ? $previous->getResponse() : null;
+
+        if (!$response instanceof ResponseInterface) {
+            return null;
+        }
+
+        $body = json_decode((string) $response->getBody());
+
+        return is_object($body) ? $body : null;
+    }
+
     public function isNotFound(): bool
     {
         return $this->status === 404;
diff --git a/src/Id/Resources/AppSubscription.php b/src/Id/Resources/AppSubscription.php
index 351f120..f4f58aa 100644
--- a/src/Id/Resources/AppSubscription.php
+++ b/src/Id/Resources/AppSubscription.php
@@ -6,6 +6,7 @@ use Anikeen\Id\Concerns\HasSubscriptionStatus;
 use Anikeen\Id\Exceptions\ApiException;
 use Anikeen\Id\Support\AppRequest;
 use DateTimeInterface;
+use InvalidArgumentException;
 use JsonSerializable;
 
 /**
@@ -73,15 +74,46 @@ class AppSubscription implements JsonSerializable
      * $startsAt (in the past) books a service that is already running, e.g. a domain registered
      * earlier: the first period runs from $startsAt, is billed now and renews when it ends.
      *
-     * @throws ApiException 409 "already_active", "billing_address_missing", "period_already_ended", …
+     * A $startsAt older than one period is refused ("period_already_ended") unless
+     * $billElapsedPeriods is set: then the periods that have already ended are billed too, together
+     * with the running one, as long as they come to at most $maxNetAmount (net, up to two
+     * decimals) and $startsAt lies within the maximum age Anikeen ID accepts (about three years).
+     * The lines land on the current month's collective order, which is charged on the 1st of the
+     * next month. $billElapsedPeriods needs both $startsAt and $maxNetAmount.
+     *
+     * @throws ApiException 409 "already_active", "billing_address_missing", "period_already_ended",
+     *     "elapsed_periods_not_allowed" (the elapsed periods can't be billed for this subscription),
+     *     "elapsed_amount_exceeded" (they come to more than $maxNetAmount; the response body has
+     *     the booked `net_amount`), …; 422 invalid input (e.g. a start older than the maximum age)
+     * @throws InvalidArgumentException $billElapsedPeriods without $startsAt or $maxNetAmount
      */
-    public function checkout(DateTimeInterface|string|null $startsAt = null): self
-    {
+    public function checkout(
+        DateTimeInterface|string|null $startsAt = null,
+        bool $billElapsedPeriods = false,
+        float|string|null $maxNetAmount = null,
+    ): self {
         if ($startsAt instanceof DateTimeInterface) {
             $startsAt = $startsAt->format(DateTimeInterface::ATOM);
         }
 
-        return $this->send('PUT', '/checkout', $startsAt === null ? null : ['starts_at' => $startsAt]);
+        if (!$billElapsedPeriods) {
+            return $this->send('PUT', '/checkout', $startsAt === null ? null : ['starts_at' => $startsAt]);
+        }
+
+        if ($startsAt === null || $startsAt === '') {
+            throw new InvalidArgumentException('Billing elapsed periods needs a start date ($startsAt).');
+        }
+
+        if ($maxNetAmount === null || $maxNetAmount === '') {
+            throw new InvalidArgumentException('Billing elapsed periods needs a maximum net amount ($maxNetAmount).');
+        }
+
+        return $this->send('PUT', '/checkout', [
+            'starts_at' => $startsAt,
+            'bill_elapsed_periods' => true,
+            // floats from calculations may carry binary noise (e.g. 59.970000000000006)
+            'max_net_amount' => is_float($maxNetAmount) ? round($maxNetAmount, 2) : $maxNetAmount,
+        ]);
     }
 
     /**
diff --git a/tests/Id/AppSubscriptionsTest.php b/tests/Id/AppSubscriptionsTest.php
index 53e41ef..8003bc9 100644
--- a/tests/Id/AppSubscriptionsTest.php
+++ b/tests/Id/AppSubscriptionsTest.php
@@ -87,6 +87,87 @@ class AppSubscriptionsTest extends TestCase
         $this->assertSame(['starts_at' => '2026-05-19T11:33:58+00:00'], $this->sentJson(1));
     }
 
+    public function test_a_checkout_without_arguments_sends_no_body(): void
+    {
+        $this->respond(201, $this->subscription());
+        $this->respond(200, $this->subscription(['active' => true]));
+
+        $this->owner()->appSubscriptions()->create(['name' => 'Server', 'unit' => 'hour', 'price' => 1])->checkout();
+
+        $request = $this->sent[1]['request'];
+        $this->assertSame('PUT /api/v1/client/subscriptions/01JSUB/checkout', $request->getMethod() . ' ' . $request->getUri()->getPath());
+        $this->assertSame('', (string) $request->getBody());
+        $this->assertFalse($request->hasHeader('Content-Type'));
+    }
+
+    public function test_a_checkout_from_a_date_sends_only_the_start(): void
+    {
+        $this->respond(201, $this->subscription());
+        $this->respond(200, $this->subscription(['active' => true]));
+
+        $this->owner()->appSubscriptions()->create(['name' => '.de TLD', 'unit' => 'year', 'price' => '5.19'])
+            ->checkout('2026-05-19T11:33:58+00:00');
+
+        $this->assertSame('{"starts_at":"2026-05-19T11:33:58+00:00"}', (string) $this->sent[1]['request']->getBody());
+    }
+
+    public function test_elapsed_periods_are_billed_up_to_the_ceiling(): void
+    {
+        $this->respond(201, $this->subscription());
+        $this->respond(200, $this->subscription(['active' => true]));
+
+        $this->owner()->appSubscriptions()->create(['name' => '.de TLD', 'unit' => 'year', 'price' => '5.19'])
+            ->checkout(new \DateTimeImmutable('2024-05-19 11:33:58', new \DateTimeZone('UTC')), billElapsedPeriods: true, maxNetAmount: '15.57');
+
+        $request = $this->sent[1]['request'];
+        $this->assertSame('PUT /api/v1/client/subscriptions/01JSUB/checkout', $request->getMethod() . ' ' . $request->getUri()->getPath());
+        $this->assertSame(['starts_at' => '2024-05-19T11:33:58+00:00', 'bill_elapsed_periods' => true, 'max_net_amount' => '15.57'], $this->sentJson(1));
+    }
+
+    public function test_a_float_ceiling_is_sent_with_two_decimals(): void
+    {
+        $this->respond(201, $this->subscription());
+        $this->respond(200, $this->subscription(['active' => true]));
+
+        $this->owner()->appSubscriptions()->create(['name' => '.de TLD', 'unit' => 'year', 'price' => 19.99])
+            ->checkout('2024-05-19', true, 19.99 * 3);
+
+        $this->assertSame(['starts_at' => '2024-05-19', 'bill_elapsed_periods' => true, 'max_net_amount' => 59.97], $this->sentJson(1));
+    }
+
+    public function test_billing_elapsed_periods_needs_a_start_and_a_ceiling(): void
+    {
+        $this->respond(201, $this->subscription());
+        $subscription = $this->owner()->appSubscriptions()->create(['name' => '.de TLD', 'unit' => 'year', 'price' => '5.19']);
+
+        foreach ([[null, '10.00'], ['2024-05-19', null], ['', '10.00'], ['2024-05-19', '']] as [$startsAt, $maxNetAmount]) {
+            try {
+                $subscription->checkout($startsAt, billElapsedPeriods: true, maxNetAmount: $maxNetAmount);
+                $this->fail('Expected an InvalidArgumentException.');
+            } catch (InvalidArgumentException) {
+            }
+        }
+
+        $this->assertCount(1, $this->sent);
+    }
+
+    public function test_an_exceeded_ceiling_carries_the_net_amount(): void
+    {
+        $this->respond(201, $this->subscription());
+        $this->respond(409, ['message' => 'The elapsed periods come to 20.76 net, more than 15.57.', 'error' => 'elapsed_amount_exceeded', 'net_amount' => '20.76']);
+
+        $subscription = $this->owner()->appSubscriptions()->create(['name' => '.de TLD', 'unit' => 'year', 'price' => '5.19']);
+
+        try {
+            $subscription->checkout('2024-05-19', billElapsedPeriods: true, maxNetAmount: '15.57');
+            $this->fail('Expected an ApiException.');
+        } catch (ApiException $e) {
+            $this->assertTrue($e->isConflict());
+            $this->assertSame('elapsed_amount_exceeded', $e->error);
+            $this->assertSame('20.76', $e->body()->net_amount);
+        }
+    }
+
     public function test_errors_carry_status_and_error_code(): void
     {
         $this->respond(409, ['message' => 'Product subscription has already been cancelled.', 'error' => 'already_cancelled']);
diff --git a/tests/Id/EndpointsTest.php b/tests/Id/EndpointsTest.php
index 8704144..7bd81bb 100644
--- a/tests/Id/EndpointsTest.php
+++ b/tests/Id/EndpointsTest.php
@@ -4,6 +4,7 @@ namespace Anikeen\Id\Tests;
 
 use Anikeen\Id\AnikeenId;
 use Anikeen\Id\Billable;
+use Anikeen\Id\Exceptions\ApiException;
 use Illuminate\Database\Eloquent\Model;
 
 /**
@@ -90,6 +91,52 @@ class EndpointsTest extends TestCase
         $this->assertSame(['email' => 'erika@example.com', 'password' => 'secret-123', 'name' => 'Erika Mustermann', 'password_confirmation' => 'secret-123'], $this->sentJson(0));
     }
 
+    public function test_create_user_can_send_an_invitation(): void
+    {
+        $this->respond(201, ['data' => ['id' => '01JNEW', 'email' => 'erika@example.com', 'created' => true, 'invitation_sent' => true]]);
+
+        $result = app(AnikeenId::class)->createUser(['name' => 'Erika Mustermann', 'email' => 'erika@example.com', 'send_invitation' => true]);
+
+        $this->assertSame('POST /api/v1/admin/users Bearer app-token', $this->sentCall(0));
+        $this->assertSame(['name' => 'Erika Mustermann', 'email' => 'erika@example.com', 'send_invitation' => true], $this->sentJson(0));
+        $this->assertTrue($result->data->created);
+        $this->assertTrue($result->data->invitation_sent);
+    }
+
+    public function test_find_user_by_email_uses_the_lookup_endpoint(): void
+    {
+        $this->respond(200, ['data' => ['id' => '01JUSER', 'name' => 'Erika', 'email' => 'erika@example.com', 'email_verified' => true, 'has_billing_address' => false]]);
+
+        $result = app(AnikeenId::class)->findUserByEmail('Erika@Example.com');
+
+        $this->assertSame('POST /api/v1/admin/users/lookup Bearer app-token', $this->sentCall(0));
+        $this->assertSame(['email' => 'Erika@Example.com'], $this->sentJson(0));
+        $this->assertSame('01JUSER', $result->data->id);
+        $this->assertFalse($result->data->has_billing_address);
+    }
+
+    public function test_find_user_by_email_returns_null_without_an_account(): void
+    {
+        $this->respond(404, ['message' => 'User not found.', 'error' => 'user_not_found']);
+
+        $this->assertNull(app(AnikeenId::class)->findUserByEmail('nobody@example.com'));
+    }
+
+    public function test_find_user_by_email_throws_other_failures(): void
+    {
+        $this->respond(404, ['message' => 'Not found.']); // e.g. an Anikeen ID without the lookup
+        $this->respond(500, ['message' => 'Server Error']);
+
+        foreach ([404, 500] as $status) {
+            try {
+                app(AnikeenId::class)->findUserByEmail('erika@example.com');
+                $this->fail('Expected an ApiException.');
+            } catch (ApiException $e) {
+                $this->assertSame($status, $e->status);
+            }
+        }
+    }
+
     public function test_email_check_and_socialite_users(): void
     {
         $this->respond(200, ['data' => ['exists' => true]]);