got it running already, but not according to the documentation i've
read, e.g. when logging in the roundcube gui, user + password will log you in but when i append the domain to the username e.g. user@sample.com , it always says login failed, is this normal, i mean logging in without the domain part? although i have not make the box public, i was able to send mails to the outside meaning postfix is working, right?
It's normal to login without the domain -- unless you house multiple mailing hosts on your server (either an alias or a virtual host), in which case you'd use the array function in the main configuration file to select a drop down of aliases, or install multiple versions of roundcube hosted on different url's (which we do at my company, actually -- I have 6 roundcubes, each with a custom skin catered to that company -- it just seems to ease confusion for my average users).
Either way, I have one on dreamhost.com and six on my work hosts, none of which we use the URL after our username, because the mail.domain.com is specified in the config (and if there's multiple, it's specified in the array option in the main config).
If you've sent mails through the outside world, and you're using your smtp.domian.com or mail.domain.com instead of the php mail function, then your postfix is working just fine:) If you have the outbound server blank, it's using php to send the mail, and will bypass postfix.
Regardless, if you can login, see your mail using your regular user name without email extension, and send mail, postfix is working just fine:)