I think what you are after is called "virtual alias" and you are on the right track when you said that these are alias in your /etc/alias file. The problem is that you need to have a directive in Postfix's main.cf to tell it to use an alias map. You first create the alias map which is just a left-hand right-hand email address list, run postmap against the list, and then restart postifx.
Here is a link
that gives some examples of setting this up for various scenarios that I think will show you how to accomplish this.