automatically create Spam mailbox

[Cliff Hayes]

I am trying to set up dovecot to automatically create a Spam mailbox for 
every user.

http://wiki2.dovecot.org/MailboxSettings shows how to do this but not 
what config file to put it in so I assume 10-mail.conf.
When I put it in there as follows...
namespace inbox {
  mailbox Spam {
    auto = create
    special_use = \Junk
  }
}

... I get the following error ...

doveconf: Fatal: Error in configuration file 
/etc/dovecot/conf.d/10-mail.conf line 107: Unknown setting: mailbox

How do I create the mailbox?