Today when I was running a workshop of alias, I got this error
postdrop: warning: unable to look up public/pickup: no such file or directory
I googled the solution out, seems need to mannually create a pipe for it.
mkfifo /var/spool/postfix/public/pickup
then the problem is solved.
After couples of tests, I figure out the reason. When I start the postfix, I HAVEN`T TURN SENDMAIL AND DO ALTERNATIVES  YET!!
So I guess because when the postfix start, the email pipe is occupied by sendmail. So when I try to use postfix to send mail, it can not use the pipe or create the pipe.
So the solution I guess is simply restart the service after alternative. Should be solved.
In the video, it did not shows up again because this time I do things IN RIGHT ORDER!
In the video, I installed a postfix client and use alias to test several emails.