Mail: Set domain for EHLO based upon the APP_URL

For #5990

app/App/Providers/AppServiceProvider.php

@@ -65,6 +65,13 @@ class AppServiceProvider extends ServiceProvider
             URL::forceScheme($isHttps ? 'https' : 'http');
         }
 
+        // Set SMTP mail driver to use a local domain matching the app domain,
+        // which helps avoid defaulting to a 127.0.0.1 domain
+        if ($appUrl) {
+            $hostName = parse_url($appUrl, PHP_URL_HOST) ?: null;
+            config()->set('mail.mailers.smtp.local_domain', $hostName);
+        }
+
         // Allow longer string lengths after upgrade to utf8mb4
         Schema::defaultStringLength(191);