Smarthost vs MX record mail delivery ?
Well, am not 100% sure about your question.
but Smarthost means , the SMTP relay server you use to send and recive your coprate emails without using your actual mail server,
example : you can have Exchange server as your coprate mail server and that server can send and recive emails through a Smarthost at your ISP without getting exposed to outside world as your email server, and outside world will see your Smarthost as your mail server, people do this for many reasons.
if your dealing with outside world using a Smarthost ,your MX record should point to that server, not to your exchange server.
if your usng hosted spam or antivirus, that means you are using an Smarthost for your email delevery,
this is a big subject and cannot explain every situation whwre you will be using smarthost and MX records.
if you can share with us , ehats is exact requierment and your setup, we can advice about your setup.
-----Cheers