diff --git a/appinfo/routes.php b/appinfo/routes.php
index bda4b2d2fe..c309bebacb 100644
--- a/appinfo/routes.php
+++ b/appinfo/routes.php
@@ -10,6 +10,11 @@
  */
 return [
 	'routes' => [
+		[
+			'name' => 'deep_link#open',
+			'url' => '/open/{messageId}',
+			'verb' => 'GET',
+		],
 		[
 			'name' => 'page#index',
 			'url' => '/',
diff --git a/lib/Controller/DeepLinkController.php b/lib/Controller/DeepLinkController.php
new file mode 100644
index 0000000000..d6827d0040
--- /dev/null
+++ b/lib/Controller/DeepLinkController.php
@@ -0,0 +1,99 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2024 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Mail\Controller;
+
+use OCA\Mail\Db\MailAccountMapper;
+use OCA\Mail\Db\MessageMapper;
+use OCA\Mail\Service\AccountService;
+use OCP\AppFramework\Controller;
+use OCP\AppFramework\Http\RedirectResponse;
+use OCP\IRequest;
+use OCP\IURLGenerator;
+use OCP\IUserSession;
+use Psr\Log\LoggerInterface;
+
+class DeepLinkController extends Controller {
+	private MailAccountMapper $mailAccountMapper;
+	private AccountService $accountService;
+	private MessageMapper $messageMapper;
+	private IURLGenerator $urlGenerator;
+	private IUserSession $userSession;
+	private LoggerInterface $logger;
+
+	public function __construct(
+		string $appName,
+		IRequest $request,
+		MailAccountMapper $mailAccountMapper,
+		AccountService $accountService,
+		MessageMapper $messageMapper,
+		IURLGenerator $urlGenerator,
+		IUserSession $userSession,
+		LoggerInterface $logger
+	) {
+		parent::__construct($appName, $request);
+		$this->mailAccountMapper = $mailAccountMapper;
+		$this->accountService = $accountService;
+		$this->messageMapper = $messageMapper;
+		$this->urlGenerator = $urlGenerator;
+		$this->userSession = $userSession;
+		$this->logger = $logger;
+	}
+
+	/**
+	 * @NoAdminRequired
+	 * @NoCSRFRequired
+	 * @PublicPage
+	 *
+	 * @param string $messageId
+	 * @return RedirectResponse
+	 */
+	public function open(string $messageId): RedirectResponse {
+		$user = $this->userSession->getUser();
+		if ($user === null) {
+			return new RedirectResponse($this->urlGenerator->linkToRouteAbsolute('core.page.login'));
+		}
+
+		$userId = $user->getUID();
+
+		try {
+			// Ensure formatting: Always wrapped in <>
+			$cleanedId = '<' . trim($messageId, '<>') . '>';
+
+			$lightAccounts = $this->mailAccountMapper->findByUserId($userId);
+			
+			foreach ($lightAccounts as $lightAccount) {
+				$accountId = $lightAccount->getId();
+				$account = $this->accountService->find($userId, $accountId);
+				$messages = $this->messageMapper->findByMessageId($account, $cleanedId);
+				
+				if (!empty($messages)) {
+					$message = $messages[0];
+					$targetId = $message->getId();
+					
+					// IMPORTANT FIX: Use 'mail.page.thread' instead of 'mail.page#thread'
+					$url = $this->urlGenerator->linkToRouteAbsolute(
+						'mail.page.thread',
+						['mailboxId' => $message->getMailboxId(), 'id' => $targetId]
+					);
+					
+					return new RedirectResponse($url);
+				}
+			}
+		} catch (\Exception $e) {
+			$this->logger->error('DeepLinkController: An unexpected error occurred.', [
+				'exception' => $e,
+				'messageId' => $messageId,
+			]);
+		}
+
+		// Fallback
+		return new RedirectResponse($this->urlGenerator->linkToRouteAbsolute('mail.page.index', []));
+	}
+}