automatically create Spam mailbox

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?