User mailbox not appearing in Sync options

Hi all,

I have a client that has EV 10.0.4 installed targeting Exchange 2007 and one 2013 box. Strange this is that when you try and sync a mailbox the mailbox does not appear. But if you choose to change the system mailbox you can see the mailbox. ALso you cannot see the mailbox in either the disable or enable mailbox options.

Any ideas?

Rob