MTA is not an agent it is a TYPE of Mail Gateway. It is also called the Next Hop email server...etc
Postfix, Exchange, Proofpoint, Ironport, and all sorts of other mail products are considered MTA's. Postfix is open source.
So when it comes to configuring one, it all depends on the SW.
" a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture."
Overall you will need to know more about Email Architectures in order to really understand this.
Good Luck,
Ronak
Please Marked solved when possible.