only register id domain if a value was set

src/Illuminate/Mail/MailServiceProvider.php

@@ -89,14 +89,16 @@ public function registerSwiftMailer()
     {
         $this->registerSwiftTransport();
 
-        Swift_DependencyContainer::getInstance()
-            ->register('mime.idgenerator.idright')
-            ->asValue($this->app->make('config')->get('mail.domain', 'swift.generated'));
-
         // Once we have the transporter registered, we will register the actual Swift
         // mailer instance, passing in the transport instances, which allows us to
         // override this transporter instances during app start-up if necessary.
         $this->app->singleton('swift.mailer', function ($app) {
+            if ($mailDomain = $app->make('config')->get('mail.domain')) {
+                Swift_DependencyContainer::getInstance()
+                                ->register('mime.idgenerator.idright')
+                                ->asValue($mailDomain);
+            }
+
             return new Swift_Mailer($app['swift.transport']->driver());
         });
     }