I installed postfix and got this error:
postdrop: warning: unable to look up public/pickup: No such file or directory.

Turns out that sendmail was previously installed and that was messing things up. I had to stop sendmail and make the appropriate directory and restart postfix.

Specifically:

mkfifo /var/spool/postfix/public/pickup
ps aux | grep mail
kill
sudo /etc/init.d/postfix restart

Test emailing
#email youremail@abc.com (enter)
subject: test (enter)
test (ctrl+d)
cc: youremail@abc.com (enter)

if your receive above then email everything is fine now! :)