feat: implement mail-login directadmin plugin

exec/lib/SourceAuthorizer.php

@@ -0,0 +1,16 @@
+<?php
+declare(strict_types=1);
+
+final class SourceAuthorizer
+{
+    public static function assertAllowed(string $sourceHost, Settings $settings): void
+    {
+        $allowed = $settings->allowedSourceHosts();
+        if ($allowed === []) {
+            throw new RuntimeException('No source host allowlist configured');
+        }
+        if (!in_array(Settings::normalizeHost($sourceHost), $allowed, true)) {
+            throw new RuntimeException('Source host is not allowed');
+        }
+    }
+}