Re: Mx records
In most domains, there is no need to create internal MX records
From my own experience when I added new domains to my Exchange, you contact your domain name holder company or log on to your DNS record management console and change the MX record for your domain name.
The usual time it takes for the change to sync around the globe DNS servers is 24h, but for us it was in few hours.
So as Ossian said, conntact the public domain name holder and ask for MX record IP change.