See if you already have postfix on the system (you could just use "find / -type f -name postfix". It should return an executable in /usr/sbin or similar if it's installed. If so, just fire it up (can't recall how to start services in Ubuntu -, maybe /etc/init.d/postfix start"?). It probably works at a basic level out of the box.
Otherwise, install with apt or apt-get.