feat: add List-Unsubscribe header to emails

framework/core/src/Mail/MailServiceProvider.php

@@ -12,8 +12,10 @@
 use Flarum\Foundation\AbstractServiceProvider;
 use Flarum\Settings\SettingsRepositoryInterface;
 use Illuminate\Contracts\Container\Container;
+use Illuminate\Contracts\Events\Dispatcher;
 use Illuminate\Contracts\Mail\Mailer as MailerContract;
 use Illuminate\Contracts\Validation\Factory;
+use Illuminate\Mail\Events\MessageSending;
 use Illuminate\Support\Arr;
 use Symfony\Component\Mailer\Transport\TransportInterface;
 
@@ -82,4 +84,9 @@ public function register(): void
 
         $this->container->alias('mailer', MailerContract::class);
     }
+
+    public function boot(Dispatcher $events): void
+    {
+        $events->listen(MessageSending::class, MutateEmail::class);
+    }
 }

framework/core/src/Mail/MutateEmail.php

@@ -0,0 +1,26 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Mail;
+
+use Illuminate\Mail\Events\MessageSending;
+
+class MutateEmail
+{
+    public function handle(MessageSending $event): bool
+    {
+        if (! empty($link = $event->data['unsubscribeLink'])) {
+            $headers = $event->message->getHeaders();
+
+            $headers->addTextHeader('List-Unsubscribe', '<'.$link.'>');
+        }
+
+        return true;
+    }
+}