I did what you said Falko.
Now it seems to work with the main email "email@example.com", this is the email address that i've add to ISPConfig,
but it is not working if a user with the same domain but a different address i.e. "firstname.lastname@example.org" purchases something!
Do I have to create every user that my client has in is mailserver??
Are we sure isn't there a solution to "route" postifix directly outside itself (localhost) to look for the user and just send the email??