"xxx" shoud be your static ip addressCode:dig mydomain.com mx
Basically, I just repeated this for billCode:host xxx.xxx.xxx.xxx
hidden correct information is just wasting the moderator time to investigate for you.
BTw, I am seek help too on this topic about reverse dns lookup, which I am still awaiting for my ISP to return my request. Also this thread may able to help other people who needed now or future, so please be nice to the helper.