diff --git a/lib/Horde/Core/Factory/Identity.php b/lib/Horde/Core/Factory/Identity.php
index 77722e86..acf7501f 100644
--- a/lib/Horde/Core/Factory/Identity.php
+++ b/lib/Horde/Core/Factory/Identity.php
@@ -57,8 +57,35 @@ public function create($user = null, $driver = null)
 
             default:
                 if (!is_null($driver)) {
-                    $class = Horde_String::ucfirst($driver) . '_Prefs_Identity';
-                    if (!class_exists($class)) {
+                    $class = null;
+                    foreach ([
+                        Horde_String::ucfirst($driver) . '_Prefs_Identity',
+                        strtoupper($driver) . '_Prefs_Identity',
+                    ] as $candidate) {
+                        if (class_exists($candidate)) {
+                            $class = $candidate;
+                            break;
+                        }
+                    }
+                    if (is_null($class)) {
+                        $fileroot = $registry->get('fileroot', $driver);
+                        $file = $fileroot
+                            ? $fileroot . '/lib/Prefs/Identity.php'
+                            : null;
+                        if ($file && is_readable($file)) {
+                            require_once $file;
+                            foreach ([
+                                Horde_String::ucfirst($driver) . '_Prefs_Identity',
+                                strtoupper($driver) . '_Prefs_Identity',
+                            ] as $candidate) {
+                                if (class_exists($candidate, false)) {
+                                    $class = $candidate;
+                                    break;
+                                }
+                            }
+                        }
+                    }
+                    if (is_null($class)) {
                         throw new Horde_Exception($driver . ' identity driver does not exist.');
                     }
                 }
diff --git a/test/Unit/Factory/IdentityTest.php b/test/Unit/Factory/IdentityTest.php
new file mode 100644
index 00000000..43c32948
--- /dev/null
+++ b/test/Unit/Factory/IdentityTest.php
@@ -0,0 +1,239 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Copyright 2026 Horde LLC (http://www.horde.org/)
+ *
+ * See the enclosed file LICENSE for license information (LGPL). If you
+ * did not receive this file, see http://www.horde.org/licenses/lgpl21.
+ *
+ * @category Horde
+ * @package  Core
+ * @license  http://www.horde.org/licenses/lgpl21 LGPL 2.1
+ */
+
+namespace Horde\Core\Test\Unit\Factory;
+
+use Horde_Exception;
+use Horde\Test\TestCase;
+use Horde_Core_Factory_Identity;
+use Horde_Core_Factory_Prefs;
+use Horde_Injector;
+use Horde_Prefs;
+use Horde_Prefs_Storage_Null;
+use Horde_Registry;
+use IMP_Mailbox;
+use PHPUnit\Framework\Attributes\CoversClass;
+
+/**
+ * Unit tests for Horde_Core_Factory_Identity.
+ *
+ * The identity factory resolves application-specific identity drivers by
+ * constructing a class name from the driver (application) name, e.g.
+ * {@see Horde_String::ucfirst()}('imp') . '_Prefs_Identity'}. Applications
+ * whose PHP class prefix is an acronym (IMP → IMP_Prefs_Identity, not
+ * Imp_Prefs_Identity) require additional resolution logic. These tests
+ * guard that behaviour and the fileroot fallback used when the class has
+ * not been autoloaded yet.
+ */
+#[CoversClass(Horde_Core_Factory_Identity::class)]
+class IdentityTest extends TestCase
+{
+    private ?Horde_Injector $injector = null;
+
+    private $registry = null;
+
+    private ?Horde_Prefs $prefs = null;
+
+    /**
+     * Prepare a minimal Horde runtime for factory calls.
+     *
+     * Sets up mocked registry and injector dependencies, plus a real
+     * Horde_Prefs instance (null storage) in $GLOBALS['prefs'] so that
+     * Horde_Core_Factory_Identity::create() can instantiate identity
+     * objects without touching a database.
+     */
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->registry = $this->getMockSkipConstructor(Horde_Registry::class);
+        $this->registry->method('getAuth')->willReturn('user@example.com');
+        $this->registry->method('getApp')->willReturn('horde');
+
+        $prefsFactory = $this->getMockSkipConstructor(Horde_Core_Factory_Prefs::class);
+        $prefsFactory->method('create')->willReturnCallback(
+            function ($app) {
+                return $this->createPrefs($app);
+            }
+        );
+
+        $this->injector = $this->getMockSkipConstructor(Horde_Injector::class);
+        $this->injector->method('getInstance')->willReturnCallback(
+            function ($class) use ($prefsFactory) {
+                return match ($class) {
+                    'Horde_Core_Factory_Prefs' => $prefsFactory,
+                    'Horde_Registry' => $this->registry,
+                    default => throw new Horde_Exception('Unexpected getInstance: ' . $class),
+                };
+            }
+        );
+
+        $this->prefs = $this->createPrefs('horde');
+        $GLOBALS['prefs'] = $this->prefs;
+        $GLOBALS['registry'] = $this->registry;
+    }
+
+    protected function tearDown(): void
+    {
+        unset($GLOBALS['prefs'], $GLOBALS['registry']);
+
+        parent::tearDown();
+    }
+
+    /**
+     * Build a Horde_Prefs object with values sufficient for identity init().
+     *
+     * IMP_Prefs_Identity::verify() reads IMP-specific preference keys during
+     * init(); those are pre-populated here when horde/imp is available.
+     *
+     * @param string $app  Application scope for the prefs object.
+     *
+     * @return Horde_Prefs  Prefs usable by Horde_Core_Prefs_Identity subclasses.
+     */
+    private function createPrefs(string $app): Horde_Prefs
+    {
+        $prefs = new Horde_Prefs($app, new Horde_Prefs_Storage_Null('user@example.com'));
+        $prefs->retrieve();
+        $prefs->changeScope($app);
+        $scope = $prefs->getScopeObject();
+        $scope->set('from_addr', 'user@example.com');
+        $scope->set('fullname', 'Test User');
+        $scope->set('identities', '');
+        $scope->set('default_identity', 0);
+        $scope->set('id', 'Default Identity');
+        $scope->set('replyto_addr', '');
+        $scope->set('alias_addr', '');
+        $scope->set('tieto_addr', '');
+        $scope->set('bcc_addr', '');
+        $scope->set('signature', '');
+        $scope->set('signature_html', '');
+        $scope->set('save_sent_mail', false);
+
+        if (class_exists('IMP_Mailbox')) {
+            $scope->set(IMP_Mailbox::MBOX_SENT, 'Sent');
+        }
+
+        return $prefs;
+    }
+
+    private function createFactory(): Horde_Core_Factory_Identity
+    {
+        return new Horde_Core_Factory_Identity($this->injector);
+    }
+
+    /**
+     * The imp driver must resolve to IMP_Prefs_Identity, not Imp_Prefs_Identity.
+     *
+     * Background: Horde_Core_Factory_Identity used to build only
+     * Horde_String::ucfirst('imp') . '_Prefs_Identity' → Imp_Prefs_Identity.
+     * The real IMP class is IMP_Prefs_Identity (all-caps acronym prefix).
+     * On a cold request, class_exists('Imp_Prefs_Identity') is false and the
+     * factory threw "{driver} identity driver does not exist", which broke
+     * IMP_Identity injection (e.g. when opening Ingo from smart mobile view).
+     *
+     * This test calls create(null, 'imp') end-to-end and asserts the returned
+     * object is an IMP_Prefs_Identity instance, proving the strtoupper()
+     * candidate in the factory resolves the correct class.
+     *
+     * Skipped when horde/imp is not installed (optional dependency).
+     */
+    public function testCreateImpDriverReturnsImpPrefsIdentity(): void
+    {
+        if (!class_exists('IMP_Prefs_Identity')) {
+            $this->markTestSkipped('horde/imp is not installed');
+        }
+
+        $identity = $this->createFactory()->create(null, 'imp');
+
+        $this->assertInstanceOf('IMP_Prefs_Identity', $identity);
+    }
+
+    /**
+     * An unknown driver must fail with a clear Horde_Exception.
+     *
+     * After trying ucfirst/strtoupper class names and an optional fileroot
+     * fallback, the factory must still reject drivers that have no identity
+     * class and no lib/Prefs/Identity.php on disk. This ensures callers
+     * (e.g. Horde_Core_Prefs_Ui) receive the same error message as before
+     * rather than a generic class-instantiation failure later.
+     *
+     * Registry::get('fileroot', 'nonexistent_app') returns null so the
+     * fileroot fallback path is also exercised and correctly skipped.
+     */
+    public function testCreateUnknownDriverThrows(): void
+    {
+        $this->expectException(Horde_Exception::class);
+        $this->expectExceptionMessage('nonexistent_app identity driver does not exist');
+
+        $this->registry->method('get')->willReturnMap([
+            ['fileroot', 'nonexistent_app', null],
+        ]);
+
+        $this->createFactory()->create(null, 'nonexistent_app');
+    }
+
+    /**
+     * Identity classes must be loadable from the application fileroot fallback.
+     *
+     * When neither Horde_String::ucfirst($driver) nor strtoupper($driver)
+     * finds an already-autoloaded class, the factory loads
+     * {fileroot}/lib/Prefs/Identity.php via require_once and re-checks both
+     * naming conventions. This mirrors production behaviour when IMP (or
+     * another app) has been registered in the registry but its libraries
+     * have not yet been touched by the autoloader.
+     *
+     * Uses a synthetic driver name (impfileroottest) and a temporary fileroot
+     * tree so the test does not depend on horde/imp and cannot conflict with
+     * IMP_Prefs_Identity. The stub class IMPFILEROOTTEST_Prefs_Identity is
+     * created on disk; the factory must require that file and instantiate it.
+     *
+     * Skipped if the synthetic class is already defined (e.g. repeated run
+     * in the same PHP process without process isolation).
+     */
+    public function testCreateLoadsIdentityClassFromApplicationFileroot(): void
+    {
+        $driver = 'impfileroottest';
+        $class = 'IMPFILEROOTTEST_Prefs_Identity';
+
+        if (class_exists($class, false)) {
+            $this->markTestSkipped($class . ' is already defined');
+        }
+
+        $fileroot = sys_get_temp_dir() . '/horde-identity-test-' . getmypid();
+        $identityDir = $fileroot . '/lib/Prefs';
+        mkdir($identityDir, 0777, true);
+
+        $file = $identityDir . '/Identity.php';
+        file_put_contents(
+            $file,
+            '<?php class ' . $class . ' extends Horde_Core_Prefs_Identity { public function init() {} }'
+        );
+
+        try {
+            $this->registry->method('get')->willReturnMap([
+                ['fileroot', $driver, $fileroot],
+            ]);
+
+            $identity = $this->createFactory()->create(null, $driver);
+
+            $this->assertInstanceOf($class, $identity);
+        } finally {
+            @unlink($file);
+            @rmdir($identityDir);
+            @rmdir($fileroot . '/lib');
+            @rmdir($fileroot);
+        }
+    }
+}