Postfix is an advanced mail server, MTA (mail transport agent). Postfix attempts to be fast, easy to administer, and secure. The outside has a definite Sendmail-ish flavor, but the inside is completely different. Funtoo/Linux providing postfix as default MTA.
# emerge postfix
At this point emerge will try to replace any alternative MTA (if any) installed on your system. It is safe to proceed.
Now you will start postfix and add it to the daemons that run on start.
# rc-update add postfix default