add_action( 'woocommerce_after_checkout_validation', 'custom_validation_email', 10, 2);
 
function custom_validation_email( $fields, $errors ){
    $domain_names = ['gmail.com', 'yandex.com', 'yahoo.com'];
    
    $valid = true;
    
    $email_domain = count(explode('@', $fields['billing_email'])) > 1 ? explode('@', $fields['billing_email'])[1] : '';
    $email_domain = strtolower($email_domain);
    
    foreach($domain_names as $domain_name) {
        if($email_domain == $domain_name) {
            $valid = false;
        }
    }
    
    if(!$valid) {
         $errors->add('validation', 'Gmail, Yandex ve Yahoo mail adreslerini kullanamazsınız.');
    }
}