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  The different parts of PostfixAdmin

/admin directory scripts are used by superadmins, also known as 'site admins'.
When you log in as a superadmin you'll be running scripts from /admin that
won't bother checking to see if you are allowed to admin for a domain.  As
a superadmin you can add and remove domains, mailboxes, aliases, domain admins,
and create more superadmins.  A superadmin will be not stopped from doing
anything, so be careful, you can delete your our login account.

login.php sorts out what type of admin you are and sends you to the right place.
The scripts in / are used by domain admins and always check to see if you are
an admin for the domain (or user within a domain) you are attempting to change.

End user scripts only do operations on the currently logged in user. 
If the end user options provide more than you want your users to have,
check the user options in config.inc.php, or, even better, edit the
user templates and remove the options.