Check the setting of postfix's mydestination parameter:

fremont postfix # postconf | grep ^mydestination
mydestination = $mydomain $myhostname localhost.$mydomain localhost

My guess is that your mydestination parameter is still set to the install default:@/etc/postfix/main.cf:

The default is $myhostname + localhost.$mydomain. On a mail domain

gateway, you should also include $mydomain.