diff --git a/lib/Horde/Core/ActiveSync/Driver.php b/lib/Horde/Core/ActiveSync/Driver.php
index aa3d81d6..a017e29c 100644
--- a/lib/Horde/Core/ActiveSync/Driver.php
+++ b/lib/Horde/Core/ActiveSync/Driver.php
@@ -3380,6 +3380,24 @@ public function meetingResponse(array $response)
             throw new Horde_ActiveSync_Exception($e);
         }
 
+        if (!empty($proposedStart) && !empty($proposedEnd)) {
+            try {
+                $this->_registry->call('calendar/storeAttendeeProposal', [
+                    $uid,
+                    $email,
+                    $proposedStart,
+                    $proposedEnd,
+                ]);
+            } catch (Horde_Exception $e) {
+                $this->_logger->warn(sprintf(
+                    'Unable to store attendee counter-proposal on calendar (uid=%s, attendee=%s): %s',
+                    $uid,
+                    $email,
+                    $e->getMessage()
+                ));
+            }
+        }
+
         if (!empty($response['sendresponse']) || !empty($proposedStart)) {
             if (!empty($response['sendresponse']) && $response['sendresponse'] !== true) {
                 $comment = $response['sendresponse']->data;
diff --git a/test/Unit/ActiveSync/DriverMeetingResponseTest.php b/test/Unit/ActiveSync/DriverMeetingResponseTest.php
new file mode 100644
index 00000000..54db032e
--- /dev/null
+++ b/test/Unit/ActiveSync/DriverMeetingResponseTest.php
@@ -0,0 +1,362 @@
+<?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.
+ *
+ * @author   Torben Dannhauer
+ * @category Horde
+ * @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\Http\ServerRequest;
+use Horde_ActiveSync;
+use Horde_ActiveSync_Exception;
+use Horde_ActiveSync_Request_MeetingResponse;
+use Horde_Core_ActiveSync_Auth;
+use Horde_Core_ActiveSync_Connector;
+use Horde_Core_ActiveSync_Driver;
+use Horde_Date;
+use Horde_Registry;
+use PHPUnit\Framework\Attributes\CoversMethod;
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+use ReflectionProperty;
+
+/**
+ * Unit tests for Horde_Core_ActiveSync_Driver::meetingResponse().
+ */
+#[CoversMethod(Horde_Core_ActiveSync_Driver::class, 'meetingResponse')]
+class DriverMeetingResponseTest extends TestCase
+{
+    use MockSkipConstructorTrait;
+
+    private array $savedGlobals = [];
+
+    protected function tearDown(): void
+    {
+        if (array_key_exists('injector', $this->savedGlobals)) {
+            $GLOBALS['injector'] = $this->savedGlobals['injector'];
+        } else {
+            unset($GLOBALS['injector']);
+        }
+
+        parent::tearDown();
+    }
+
+    public function testMeetingResponseStoresAttendeeProposalWhenProposedTimesArePresent(): void
+    {
+        $this->requireMeetingResponseDependencies();
+        $this->installIdentityInjector();
+
+        $folderId = 'INBOX';
+        $requestId = '42';
+        $uid = 'server-event-uid';
+        $expectedStart = new Horde_Date('2026-06-19T12:00:00.000Z', 'UTC');
+        $expectedEnd = new Horde_Date('2026-06-19T13:00:00.000Z', 'UTC');
+
+        $registry = $this->getMockSkipConstructor(Horde_Registry::class);
+        $registry->expects($this->once())
+            ->method('call')
+            ->with(
+                'calendar/storeAttendeeProposal',
+                $this->callback(static function (array $args) use ($uid, $expectedStart, $expectedEnd): bool {
+                    return count($args) === 4
+                        && $args[0] === $uid
+                        && $args[1] === 'attendee@example.test'
+                        && $args[2] instanceof Horde_Date
+                        && $args[2]->timestamp === $expectedStart->timestamp
+                        && $args[3] instanceof Horde_Date
+                        && $args[3]->timestamp === $expectedEnd->timestamp;
+                })
+            );
+
+        $imap = $this->getMockSkipConstructor('Horde_ActiveSync_Imap_Adapter');
+        $imap->expects($this->once())
+            ->method('getImapMessage')
+            ->with($folderId, $requestId)
+            ->willReturn($this->createImapMessage(
+                $requestId,
+                $this->buildVcalendar()
+            ));
+        $imap->expects($this->never())->method('deleteMessages');
+
+        $connector = $this->getMockSkipConstructor(Horde_Core_ActiveSync_Connector::class);
+        $connector->expects($this->once())
+            ->method('calendar_import_vevent')
+            ->willReturn($uid);
+
+        $driver = $this->createDriver($registry, $imap, $connector);
+
+        $this->expectException(Horde_ActiveSync_Exception::class);
+
+        $driver->meetingResponse([
+            'folderid' => $folderId,
+            'requestid' => $requestId,
+            'response' => Horde_ActiveSync_Request_MeetingResponse::RESPONSE_ACCEPTED,
+            'proposedstarttime' => '2026-06-19T12:00:00.000Z',
+            'proposedendtime' => '2026-06-19T13:00:00.000Z',
+        ]);
+    }
+
+    public function testMeetingResponseSkipsAttendeeProposalWhenCounterProposalsAreDisallowed(): void
+    {
+        $this->requireMeetingResponseDependencies();
+        $this->installIdentityInjector();
+
+        $folderId = 'INBOX';
+        $requestId = '42';
+        $uid = 'server-event-uid';
+
+        $registry = $this->getMockSkipConstructor(Horde_Registry::class);
+        $registry->expects($this->never())->method('call');
+
+        $imap = $this->getMockSkipConstructor('Horde_ActiveSync_Imap_Adapter');
+        $imap->expects($this->once())
+            ->method('getImapMessage')
+            ->with($folderId, $requestId)
+            ->willReturn($this->createImapMessage(
+                $requestId,
+                $this->buildVcalendar(['X-MS-DISALLOW-COUNTER:TRUE'])
+            ));
+        $imap->expects($this->once())
+            ->method('deleteMessages')
+            ->with([$requestId], $folderId);
+
+        $connector = $this->getMockSkipConstructor(Horde_Core_ActiveSync_Connector::class);
+        $connector->expects($this->once())
+            ->method('calendar_import_vevent')
+            ->willReturn($uid);
+
+        $driver = $this->createDriver($registry, $imap, $connector);
+
+        $this->assertSame($uid, $driver->meetingResponse([
+            'folderid' => $folderId,
+            'requestid' => $requestId,
+            'response' => Horde_ActiveSync_Request_MeetingResponse::RESPONSE_ACCEPTED,
+            'proposedstarttime' => '2026-06-19T12:00:00.000Z',
+            'proposedendtime' => '2026-06-19T13:00:00.000Z',
+        ]));
+    }
+
+    public function testMeetingResponseSkipsAttendeeProposalWhenProposedStartTimeIsInvalid(): void
+    {
+        $this->requireMeetingResponseDependencies();
+        $this->installIdentityInjector();
+
+        $folderId = 'INBOX';
+        $requestId = '42';
+        $uid = 'server-event-uid';
+
+        $registry = $this->getMockSkipConstructor(Horde_Registry::class);
+        $registry->expects($this->never())->method('call');
+
+        $imap = $this->getMockSkipConstructor('Horde_ActiveSync_Imap_Adapter');
+        $imap->expects($this->once())
+            ->method('getImapMessage')
+            ->with($folderId, $requestId)
+            ->willReturn($this->createImapMessage(
+                $requestId,
+                $this->buildVcalendar()
+            ));
+        $imap->expects($this->once())
+            ->method('deleteMessages')
+            ->with([$requestId], $folderId);
+
+        $connector = $this->getMockSkipConstructor(Horde_Core_ActiveSync_Connector::class);
+        $connector->expects($this->once())
+            ->method('calendar_import_vevent')
+            ->willReturn($uid);
+
+        $driver = $this->createDriver($registry, $imap, $connector);
+
+        $this->assertSame($uid, $driver->meetingResponse([
+            'folderid' => $folderId,
+            'requestid' => $requestId,
+            'response' => Horde_ActiveSync_Request_MeetingResponse::RESPONSE_ACCEPTED,
+            'proposedstarttime' => 'not-a-date',
+            'proposedendtime' => '2026-06-19T13:00:00.000Z',
+        ]));
+    }
+
+    private function requireMeetingResponseDependencies(): void
+    {
+        foreach ([
+            'Horde_ActiveSync_State_Sql',
+            'Horde_ActiveSync_Imap_Adapter',
+            'Horde_ActiveSync_Request_MeetingResponse',
+            'Horde_Icalendar',
+            'Horde_Date',
+        ] as $className) {
+            if (!class_exists($className)) {
+                $this->markTestSkipped(sprintf('%s not available', $className));
+            }
+        }
+    }
+
+    private function createDriver(
+        Horde_Registry $registry,
+        object $imap,
+        Horde_Core_ActiveSync_Connector $connector,
+    ): Horde_Core_ActiveSync_Driver {
+        $driver = new Horde_Core_ActiveSync_Driver([
+            'connector' => $connector,
+            'auth' => $this->getMockSkipConstructor(Horde_Core_ActiveSync_Auth::class),
+            'serverrequest' => new ServerRequest('POST', '/'),
+            'registry' => $registry,
+            'state' => $this->createDriverStateMock(),
+            'imap' => $imap,
+        ]);
+
+        $this->setProperty($driver, '_user', 'attendee');
+        $this->setProperty($driver, '_version', Horde_ActiveSync::VERSION_SIXTEENONE);
+        $this->setProperty($driver, '_logger', new class {
+            public function err(string $message, ?string $priority = null): void
+            {
+            }
+
+            public function meta(string $message): void
+            {
+            }
+
+            public function notice(string $message): void
+            {
+            }
+
+            public function warn(string $message): void
+            {
+            }
+        });
+
+        return $driver;
+    }
+
+    private function createDriverStateMock(): MockObject
+    {
+        $state = $this->getMockSkipConstructor('Horde_ActiveSync_State_Sql');
+        $state->expects($this->once())->method('setLogger');
+        $state->expects($this->once())->method('setBackend');
+
+        return $state;
+    }
+
+    private function installIdentityInjector(string $email = 'attendee@example.test'): void
+    {
+        if (array_key_exists('injector', $GLOBALS)) {
+            $this->savedGlobals['injector'] = $GLOBALS['injector'];
+        }
+
+        $identity = new class ($email) {
+            public function __construct(private readonly string $email)
+            {
+            }
+
+            public function getValue(string $key): ?string
+            {
+                return $key === 'from_addr' ? $this->email : null;
+            }
+
+            public function getFromAddress(): string
+            {
+                return $this->email;
+            }
+        };
+
+        $identityFactory = new class ($identity) {
+            public function __construct(private readonly object $identity)
+            {
+            }
+
+            public function create(?string $user = null): object
+            {
+                return $this->identity;
+            }
+        };
+
+        $GLOBALS['injector'] = new class ($identityFactory) {
+            public function __construct(private readonly object $identityFactory)
+            {
+            }
+
+            public function getInstance(string $class): object
+            {
+                if ($class === 'Horde_Core_Factory_Identity') {
+                    return $this->identityFactory;
+                }
+
+                throw new \RuntimeException(sprintf('Unexpected injector lookup: %s', $class));
+            }
+        };
+    }
+
+    /**
+     * @return array<string, object>
+     */
+    private function createImapMessage(string $requestId, string $ical): array
+    {
+        $part = new class ($ical) {
+            public function __construct(private readonly string $ical)
+            {
+            }
+
+            public function getContents(): string
+            {
+                return $this->ical;
+            }
+
+            public function getCharset(): string
+            {
+                return 'UTF-8';
+            }
+        };
+
+        $message = new class ($part) {
+            public function __construct(private readonly object $part)
+            {
+            }
+
+            public function hasiCalendar(): object
+            {
+                return $this->part;
+            }
+        };
+
+        return [$requestId => $message];
+    }
+
+    /**
+     * @param list<string> $extraEventLines
+     */
+    private function buildVcalendar(array $extraEventLines = []): string
+    {
+        return implode("\r\n", array_merge([
+            'BEGIN:VCALENDAR',
+            'VERSION:2.0',
+            'PRODID:-//Horde//Core Test//EN',
+            'BEGIN:VEVENT',
+            'UID:test-event-uid',
+            'DTSTART:20260619T100000Z',
+            'DTEND:20260619T110000Z',
+            'ATTENDEE:mailto:attendee@example.test',
+        ], $extraEventLines, [
+            'END:VEVENT',
+            'END:VCALENDAR',
+            '',
+        ]));
+    }
+
+    private function setProperty(object $object, string $property, mixed $value): void
+    {
+        $reflection = new ReflectionProperty($object, $property);
+        $reflection->setValue($object, $value);
+    }
+}