MTA stands for „mail transfer agent“. Popular ones are sendmail, exim and postfix.
If you want to know what MTA is running locally you can check which program listens on port 25:
lsof -i :25
You might also telnet on port 25 and let the MTA himself respond who he is:
telnet localhost 25
For configuring exim4 the preferred way is to use the graphical interface, call it with: