Here’s a simple way to change your default MTA on CentOS:

# alternatives --config mta

There are 2 programs which provide 'mta'.

  Selection    Command
-----------------------------------------------
*+ 1           /usr/sbin/sendmail.sendmail
   2           /usr/sbin/sendmail.ssmtp

Enter to keep the current selection[+], or type selection number: 2

# sendmail -V
sSMTP 2.61 (Not sendmail at all)