fix: harden mx helper and cleanup flow

exec/lib/Settings.php

@@ -105,6 +105,16 @@ final class Settings
         return rtrim($this->getString('MAIL_LOGIN_TARGET_BASE_URL'), '/');
     }
 
+    public function defaultEnable(): bool
+    {
+        return in_array(strtolower($this->getString('DEFAULT_ENABLE', 'true')), ['1', 'true', 'yes', 'on'], true);
+    }
+
+    public function method(): string
+    {
+        return $this->getString('MAIL_LOGIN_METHOD', 'login_url');
+    }
+
     public function redirectUrl(): string
     {
         return $this->getString('MAIL_LOGIN_REDIRECT_URL', '/');
@@ -229,6 +239,12 @@ final class Settings
         if (!in_array($this->getString('USER_IP_BOUND', ''), ['0', '1'], true)) {
             throw new InvalidArgumentException('USER_IP_BOUND must be 0 or 1');
         }
+        if (!in_array(strtolower($this->getString('DEFAULT_ENABLE', 'true')), ['0', '1', 'true', 'false', 'yes', 'no', 'on', 'off'], true)) {
+            throw new InvalidArgumentException('DEFAULT_ENABLE must be a boolean value');
+        }
+        if ($this->method() !== 'login_url') {
+            throw new InvalidArgumentException('MAIL_LOGIN_METHOD must be login_url');
+        }
         if (!in_array($this->clientIpMode(), ['direct', 'trusted_proxy'], true)) {
             throw new InvalidArgumentException('MAIL_LOGIN_CLIENT_IP_MODE must be direct or trusted_proxy');
         }