Can't get postfix aliases to work from /etc/aliases

Question

Ugh.  Things were going so well.  I just needed to set up a few quick mailing lists and I could open the server for testing, but I can't seem to get aliases to work.  This is my first go at postfix, so it's possible I'm misunderstanding aliases at a fundamental level here...
Here's what I want to do-- I want to have messages sent to one address ([email protected]) delivered to multple addresses ([email protected]) and ([email protected]).  domainname.tld is the hostname of my server machine.
I've created an alias of this form:
baz:  [email protected], [email protected]
I started by putting it in '/Library/Server/Mail/Config/postfix/mailing_list_aliases', but when that didn't work, I moved it to /etc/aliases, and when that didn't work I simplified the alias to:
baz: [email protected]
I've done newaliases without error, and a postfix reload though I'm not sure that second one is necessary.
Originally, I was getting the messages bounced saying that [email protected] is unknown in the local recipient table.  Now with the single address mapping in the /etc/aliases file, it's telling me that [email protected], mapped from [email protected], is "user unknown".  (Ok, there was probably a more grammatically correct way to form that sentence...).
In Mail, [email protected] is recognized and made a hot link, which creates a message to that address when I click it.  If I send that message, it's delivered without a problem.
I've run out of ideas, and internet research turned up a solution yet-- which probably means I really don't know what question to ask.

Answer 1

New developments:
I got this working thanks to Jaydisc's comment elsewhere:
"All of this is doable by manipulating postfix underneatht the hood. I haven't seen what file Mountain Lion specifically creates when you turn on virtual hosting, but traditionally these kind of mappings are stored in /etc/postfix/virtual. You need to postmap the file after making changes and restart postfix.
Jonathan, mailman is seemingly no longer included in OS X Server whicih would ahve been perfect. That means you need to manually build your list in /etc/postfix/virtual or install a 3rd party solution as far as I'm aware. In /etc/postfix/virtual, you could so something like:
[email protected]               [email protected], [email protected], [email protected]
Levin, you don't want to do a catch-all, but if you insist, assuming it's with a virtual domain, you'd put in
@domein.com                   [email protected]
When done, run:
postmap /etc/postfix/virtual
postfix reload
Do all of this as root or using the sudo command. If you don't know what I'm talking about, you probably shouldn't do it."
I edited /etc/postfix/virtual via the command line, like:
sudo nano /etc/postfix/virtual
And add the group and the users like in the example above.
This was new for me, but in order to get my own mailing lists working together with Prevent mail server spoofing I had to remove mailman and use the option you desired instead I'm happy you asked for more!