On our campaigns I have noticed that the analytics are wrong. The number of recipients that have opened our email is not always shown properly. I tried verifying it with our internal emails as well. Any help would be appreciated.
The problem is because your SSL certificate is invalid, so opens and clicks can't be tracked properly.
You need to install a valid SSL certificate if you want to use 'https' for your Sendy installation.
Or don't use 'https' at all if you can't install a valid SSL certificate.