I'm going to veto this. I get the general idea, but this is a huge 
amount of complexity and even then you're not going to make things 
graceful for most users.

I could imagine that:

- a message is displayed some minutes before system shutdown in header 
or footer of each page

- no further logins are accepted

- no new e-mails or calendar entries are been processed

- opened pop or imap connections are closed down

- and finally each link of still logged in users leads to the logout page



Btw. a status line in which system messages for ordinary users could 
be displayed, might be a good idea, too

How do you log out users "smoothly"?

I would like to have the administrator's option to "disable login", so 
that users that are logged in are logged out smoothly and no other 
users except by administrators are let into the horde system.

That would be a good solution to realize system maintainance.