Stop Home Directory From Appearing in User's IMAP client's Folder List

imap4d version 3.1.1

In imap4d I have a namespace block:

namespace personal {
  prefix "" {
    directory "$home/mail";
    mailbox-type "mbox";
  }
}

When I use my imap client (Thunderbird 60.4.0), I'm seeing my entire
home directory in my folder list.

I've misunderstood something.

Thanks.
Ivan.