After installing postfix I started getting the error:

postdrop: warning: unable to look up public/pickup: No such file or directory.

This usually occurs when sendmail is installed and causing conflict with postfix.

To fix this you must run the following commands.

sudo service sendmail stop
sudo service sendmail disable ## to stop sendmail from starting with system

mkfifo /var/spool/postfix/public/pickup ## create the missing postfix directory

sudo service postfix restart

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