postfix received by hostname instead of domain?

Emails sent from postfix have the following headers;

Received: from host.server.com ([x.x.x.x]) by x.x.x with bizsmtp id xxx; Tue, 01 Sep 2015 10:32:53 +0200
Received: by host.server.com (Postfix, from userid 48) id DD575AA; Tue,  1 Sep 2015 04:32:52 -0400 (EDT)

However, I want this to show the domain name instead since this server hosts multiple sites.

I changed the postfix configuration to say :

myorigin = $mydomain

Note that $mydomain is not defined there. In the comments above this it states:

# The myorigin parameter specifies the domain that locally-posted
# mail appears to come from. The default is to append $myhostname,
# which is fine for small sites.  If you run a domain with multiple
# machines, you should (1) change this to $mydomain and (2) set up
# a domain-wide alias database that aliases each user to
# user@that.users.mailhost.

So I added an alias to /etc/aliases:

username:     info@mydomain.com

and restarted postfix.

However, mail is still being sent via host.server.com

What am I doing wrong?

I assume that mail sent via PHP is going through the correct username, and not some default one? PHP is set up as CGI, default install as virtualmin defined, nothing changed there.

Any help?

thx