QMail won't accept authentication

My goal with QMail is to allow all users of my virtual hosts (set up with plesk) to send email to anyone. I initially thought that I would have to do the tcpserver thing and enable my domains, but it now appears that the users would have to be physically logged on to the server machine for that to work (which is not the case), so I am trying to go back to the standard setup with qmail-smtp and turning on the "Authorization is required" option and SMTP checkbox in the plesk control panel.

When I do this, and have the username and password set in my email program, it comes back saying that the server doesn't accept authentication. I know that if I don't have authentication turned on and don't have rcpthosts, that the server would be left as an open relay, so I can't do that.

Can anyone shed any light on why the authentication would not be working, or suggest another way of securely achieving my goal?

Thanks

I found that the SMTP authentication was not included - have now rebuilt the entire thing, but now it says that the password is rejected. Any assistance on tracking that down would be greatly appreciated.

Did you add the user and password that you are trying?
Also,
Are you trying to login via IMAP in a browser, or POP3 via mail client? If POP3 did you setup the acct for login?