bbPress.org

use this in your functions file and change the address to what you want

add_filter('bbp_get_do_not_reply_address','my_bbp_no_reply_email');
function no_reply_email(){
    $email = 'noreply@yourdomain.com'; // any email you want
    return $email;
}

you can change that default to email address as well with :

add_filter('bbp_subscription_to_email','my_bbp_subscription_to_email');
function my_bbp_subscription_to_email(){
    $email = 'noreply@yourdomain.com'; // any email you want
    return $email;
}