i use mail server of yandex but i get this error when i enter mail.kvadrat.az
Welcome to the Cloudflare Community. 
Only the operator of the mail.kvadrat.az site will be able to fix this. If it is your site, make sure the certificate on the origin server is valid and issued by a recognized public certificate authority. If you are not the site operator you will need to contact them, so they can fix the certificate on their server.