With openpos, cause wordpress user have email is require. So, with those customer add via openpos but have no email, openpos auto generate a email with email domain @open-pos.com . to change this default , shop owner can change it by add and custom this code to your-theme/functions.php or snippets plugin

function custom_customer_demo_domain($demo_email){
     return '@your-custom-domain.com';
}
add_filter('op_customer_demo_domain','custom_customer_demo_domain',10,1);