Sometimes you may have infrastructure on your end that will block MageMail API requests – for example a firewall or Cloudflare setup.
In the event that API requests are being blocked, you can whitelist the MageMail IP Addresses: [please log in].
You may also need to whitelist the API route that’s being requested:
https://www.example.com/index.php/magemail/api/query/route
https://www.example.com/index.php/magemail/api/insert/route