Running a script for email received

[Ori Idan]

I am trying to run a script when an email is received on my server.
I tried what described here:
http://blog.thecodingmachine.com/content/triggering-php-script-when-your-postfix-server-receives-mail

But it invoked the script for all mails not mail to a specific user
I tried the simplest thing of /etc/alias
But got error: Recipient address rejected: User unknown in virtual alias
table

Does anyone have any idea how to run a script for mail sent to a specific
user?

-- 
Ori Idan
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