diff --git a/lib/Horde/Core/ActiveSync/Driver.php b/lib/Horde/Core/ActiveSync/Driver.php
index e0c70fc6..25ade46b 100644
--- a/lib/Horde/Core/ActiveSync/Driver.php
+++ b/lib/Horde/Core/ActiveSync/Driver.php
@@ -1,5 +1,6 @@
 <?php
 
+use Horde\Exception\InvalidDataThrowable;
 use Psr\Http\Message\ServerRequestInterface;
 
 /**
@@ -2478,6 +2479,11 @@ public function moveMessage($folderid, array $ids, $newfolderid)
      *         if adding Draft email message. @since 2.28.0
      *  - conversationindex: (integer) The conversation index value if adding
      *         Draft email message @since 2.28.0
+     *
+     *   On failure: false for a server-side error the client may retry, or
+     *   an array with only an 'error' key holding the
+     *   Horde_ActiveSync_Request_Sync::STATUS_* code to report for the item
+     *   (STATUS_INVALID when the backend rejected the client data).
      */
     public function changeMessage($folderid, $id, Horde_ActiveSync_Message_Base $message, $device)
     {
@@ -2513,9 +2519,7 @@ public function changeMessage($folderid, $id, Horde_ActiveSync_Message_Base $mes
                     try {
                         $results = $this->_connector->calendar_import($message, $server_id);
                     } catch (Horde_Exception $e) {
-                        $this->_logger->err($e->getMessage());
-                        $this->_endBuffer();
-                        return false;
+                        return $this->_importFailure($e, $folderid, $id);
                     }
                     $stat = [
                         'mod' => $this->getSyncStamp($folderid),
@@ -2530,9 +2534,7 @@ public function changeMessage($folderid, $id, Horde_ActiveSync_Message_Base $mes
                     try {
                         $results = $this->_connector->calendar_replace($id, $message, $server_id);
                     } catch (Horde_Exception $e) {
-                        $this->_logger->err($e->getMessage());
-                        $this->_endBuffer();
-                        return false;
+                        return $this->_importFailure($e, $folderid, $id);
                     }
                     $stat = $this->_smartStatMessage($folderid, $id, false);
                     // @todo Remove this check in H6, when the API always returns.
@@ -2549,18 +2551,14 @@ public function changeMessage($folderid, $id, Horde_ActiveSync_Message_Base $mes
                     try {
                         $id = $this->_connector->contacts_import($message, $server_id);
                     } catch (Horde_Exception $e) {
-                        $this->_logger->err($e->getMessage());
-                        $this->_endBuffer();
-                        return false;
+                        return $this->_importFailure($e, $folderid, $id);
                     }
                     $stat = ['mod' => $this->getSyncStamp($folderid), 'id' => $id, 'flags' => 1];
                 } else {
                     try {
                         $this->_connector->contacts_replace($id, $message);
                     } catch (Horde_Exception $e) {
-                        $this->_logger->err($e->getMessage());
-                        $this->_endBuffer();
-                        return false;
+                        return $this->_importFailure($e, $folderid, $id);
                     }
                     $stat = $this->_smartStatMessage($folderid, $id, false);
                 }
@@ -2571,18 +2569,14 @@ public function changeMessage($folderid, $id, Horde_ActiveSync_Message_Base $mes
                     try {
                         $id = $this->_connector->tasks_import($message, $server_id);
                     } catch (Horde_Exception $e) {
-                        $this->_logger->err($e->getMessage());
-                        $this->_endBuffer();
-                        return false;
+                        return $this->_importFailure($e, $folderid, $id);
                     }
                     $stat = ['mod' => $this->getSyncStamp($folderid), 'id' => $id, 'flags' => 1];
                 } else {
                     try {
                         $this->_connector->tasks_replace($id, $message);
                     } catch (Horde_Exception $e) {
-                        $this->_logger->err($e->getMessage());
-                        $this->_endBuffer();
-                        return false;
+                        return $this->_importFailure($e, $folderid, $id);
                     }
                     $stat = $this->_smartStatMessage($folderid, $id, false);
                 }
@@ -2593,18 +2587,14 @@ public function changeMessage($folderid, $id, Horde_ActiveSync_Message_Base $mes
                     try {
                         $id = $this->_connector->notes_import($message, $server_id);
                     } catch (Horde_Exception $e) {
-                        $this->_logger->err($e->getMessage());
-                        $this->_endBuffer();
-                        return false;
+                        return $this->_importFailure($e, $folderid, $id);
                     }
                     $stat = ['mod' => $this->getSyncStamp($folderid), 'id' => $id, 'flags' => 1];
                 } else {
                     try {
                         $this->_connector->notes_replace($id, $message);
                     } catch (Horde_Exception $e) {
-                        $this->_logger->err($e->getMessage());
-                        $this->_endBuffer();
-                        return false;
+                        return $this->_importFailure($e, $folderid, $id);
                     }
                     $stat = $this->_smartStatMessage($folderid, $id, false);
                 }
@@ -4661,6 +4651,39 @@ protected function _endBuffer()
         }
     }
 
+    /**
+     * Turn an exception raised by a PIM backend while importing a client
+     * change into the changeMessage() failure result.
+     *
+     * A backend that rejected the client's data (InvalidDataThrowable, e.g.
+     * an attribute over its configured length) is reported as STATUS_INVALID:
+     * the item itself is unacceptable and the client must not resend it on
+     * every Sync. Any other failure stays a generic server error (false),
+     * which the client may retry.
+     *
+     * @param Horde_Exception $e  The backend exception.
+     * @param string $folderid    The folder the change was addressed to.
+     * @param string|null $id     The server id of the changed item, null for
+     *                            an addition.
+     *
+     * @return array|boolean  @see self::changeMessage()
+     */
+    protected function _importFailure(Horde_Exception $e, $folderid, $id)
+    {
+        $this->_endBuffer();
+        if ($e instanceof InvalidDataThrowable) {
+            $this->_logger->warn(sprintf(
+                'Rejected client change for %s in folder %s: %s',
+                $id ?: 'new item',
+                $folderid,
+                $e->getMessage()
+            ));
+            return ['error' => Horde_ActiveSync_Request_Sync::STATUS_INVALID];
+        }
+        $this->_logger->err($e->getMessage());
+        return false;
+    }
+
     /**
      * Removes the beginning/ending delimiters from the certificate.
      *
diff --git a/test/Unit/ActiveSync/DriverChangeMessageStatusTest.php b/test/Unit/ActiveSync/DriverChangeMessageStatusTest.php
new file mode 100644
index 00000000..3c25593f
--- /dev/null
+++ b/test/Unit/ActiveSync/DriverChangeMessageStatusTest.php
@@ -0,0 +1,131 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Copyright 2026 The Horde Project (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.
+ *
+ * @author   Torben Dannhauer <torben@dannhauer.de>
+ * @category Horde
+ * @copyright 2026 The Horde Project
+ * @license  http://www.horde.org/licenses/lgpl21 LGPL
+ * @package  Core
+ * @subpackage UnitTests
+ */
+
+namespace Horde\Core\Test\Unit\ActiveSync;
+
+use Horde\Core\Test\Support\MockSkipConstructorTrait;
+use Horde\Exception\InvalidDataThrowable;
+use Horde\Http\ServerRequest;
+use Horde_ActiveSync_Message_Contact;
+use Horde_ActiveSync_Request_Sync;
+use Horde_Core_ActiveSync_Auth;
+use Horde_Core_ActiveSync_Connector;
+use Horde_Core_ActiveSync_Driver;
+use Horde_Exception;
+use Horde_Registry;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Unit tests for the failure result of
+ * Horde_Core_ActiveSync_Driver::changeMessage().
+ *
+ * A PIM backend that rejects the client's data (InvalidDataThrowable, e.g.
+ * Turba refusing an attribute over its configured length) must be reported
+ * as STATUS_INVALID so the client stops resending the item; any other
+ * backend failure stays a retryable false.
+ */
+#[CoversClass(Horde_Core_ActiveSync_Driver::class)]
+class DriverChangeMessageStatusTest extends TestCase
+{
+    use MockSkipConstructorTrait;
+
+    public function testRejectedClientDataIsReportedAsStatusInvalid(): void
+    {
+        $connector = $this->getMockSkipConstructor(Horde_Core_ActiveSync_Connector::class);
+        $connector->expects($this->once())
+            ->method('contacts_replace')
+            ->willThrowException($this->invalidDataException());
+
+        $result = $this->driver($connector)->changeMessage(
+            Horde_Core_ActiveSync_Driver::CONTACTS_FOLDER_UID,
+            'contact-1',
+            new Horde_ActiveSync_Message_Contact(),
+            null
+        );
+
+        $this->assertSame(
+            ['error' => Horde_ActiveSync_Request_Sync::STATUS_INVALID],
+            $result
+        );
+    }
+
+    public function testRejectedClientDataOnAddIsReportedAsStatusInvalid(): void
+    {
+        $connector = $this->getMockSkipConstructor(Horde_Core_ActiveSync_Connector::class);
+        $connector->expects($this->once())
+            ->method('contacts_import')
+            ->willThrowException($this->invalidDataException());
+
+        $result = $this->driver($connector)->changeMessage(
+            Horde_Core_ActiveSync_Driver::CONTACTS_FOLDER_UID,
+            null,
+            new Horde_ActiveSync_Message_Contact(),
+            null
+        );
+
+        $this->assertSame(
+            ['error' => Horde_ActiveSync_Request_Sync::STATUS_INVALID],
+            $result
+        );
+    }
+
+    public function testOtherBackendFailureStaysRetryableFalse(): void
+    {
+        $connector = $this->getMockSkipConstructor(Horde_Core_ActiveSync_Connector::class);
+        $connector->expects($this->once())
+            ->method('contacts_replace')
+            ->willThrowException(new Horde_Exception('Server error when saving data.'));
+
+        $result = $this->driver($connector)->changeMessage(
+            Horde_Core_ActiveSync_Driver::CONTACTS_FOLDER_UID,
+            'contact-1',
+            new Horde_ActiveSync_Message_Contact(),
+            null
+        );
+
+        $this->assertFalse($result);
+    }
+
+    private function invalidDataException(): Horde_Exception
+    {
+        return new class ('Home Postal Code: Value is over the maximum length of 10.') extends Horde_Exception implements InvalidDataThrowable {
+        };
+    }
+
+    private function driver(Horde_Core_ActiveSync_Connector $connector): Horde_Core_ActiveSync_Driver
+    {
+        if (!class_exists('Horde_ActiveSync_State_Sql')) {
+            $this->markTestSkipped('horde/activesync not available');
+        }
+
+        $state = $this->getMockSkipConstructor('Horde_ActiveSync_State_Sql');
+        $auth = $this->getMockSkipConstructor(Horde_Core_ActiveSync_Auth::class);
+        $auth->expects($this->never())->method($this->anything());
+        $registry = $this->getMockSkipConstructor(Horde_Registry::class);
+        $registry->expects($this->never())->method($this->anything());
+
+        return new Horde_Core_ActiveSync_Driver([
+            'connector' => $connector,
+            'auth' => $auth,
+            'serverrequest' => new ServerRequest('POST', '/'),
+            'registry' => $registry,
+            'state' => $state,
+        ]);
+    }
+}