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