Disable user account

I've looked around in my ESSG and on these forums, but I just can't understand what's the difference between a newly created user (one that gets displayed in red in the user list) and an already existing user whose password has been reset to the default one.

A newly created user can't access his mailbox, while an active user can, even if his password is reset to the default one; changing to 'no' the PasswordSet value in the account database reddens his name in the user list, but he can still access to his mail.

Does anybody tell me how a user can be reverted to a "newly created and still not enabled" state, please?

Thanks

--

Pierluigi Miranda