You probably need to set "allowallsubscribe: 1" on the servers in your
murder.
>From the imapd.conf manpage:
     allowallsubscribe: 0
          Allow  subscription  to  nonexistent  mailboxes.   This
          option is typically used on backend servers in a Murder
          so that users can subscribe  to  mailboxes  that  don't
          reside on their "home" server.
-jeaton