Send email via ajax

You need to allow AJAX requests for guests too. Usually when a WordPress AJAX call returns 0, it means that there are no capabilities for the user to perform that action.

Also, your debug info showed Guest.

Alter your function to include the nopriv AJAX for guests:

add_action('wp_ajax_nopriv_fahadsending_mail', 'fahadsending_mail');
add_action('wp_ajax_fahadsending_mail', 'fahadsending_mail');

function fahadsending_mail(){

    $to = "[email protected]";
    $subject = "Donation";
    $message = "message message message message message message message "; 

    if( wp_mail($to, $subject, $message) ){
        echo "mail sent";
    } else {
        echo "mail not sent";
    }

    die(); // never forget to die() your AJAX reuqests

}