Mail drivers: Separate definition from validation

src/Mail/SendmailDriver.php

@@ -10,6 +10,8 @@
 namespace Flarum\Mail;
 
 use Flarum\Settings\SettingsRepositoryInterface;
+use Illuminate\Contracts\Validation\Factory;
+use Illuminate\Support\MessageBag;
 use Swift_SendmailTransport;
 use Swift_Transport;
 
@@ -20,6 +22,16 @@ class SendmailDriver implements DriverInterface
         return [];
     }
 
+    public function validate(SettingsRepositoryInterface $settings, Factory $validator): MessageBag
+    {
+        return new MessageBag;
+    }
+
+    public function canSend(): bool
+    {
+        return true;
+    }
+
     public function buildTransport(SettingsRepositoryInterface $settings): Swift_Transport
     {
         return new Swift_SendmailTransport;