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