A user of ours is unable to search ALL sub directories under the Inbox of his
Seconday Mailbox. The "search subfolders" tickbox is simply greyed out.

I on the other hand, CAN search sub-directories on my secondary mailbox (not
the same mailbox) without a problem. Is there an option associated with his
(secondary) mailbox, or is there some form of corruption?

The bizarre thing is the owner of this mailbox can search all folders
despite the fact it's her secondary mailbox as well. As a test I gave the
user full owner rights to this mailbox but is still unable to perform a full
search. Also tried the /cleanprofile and /resetfolders switch - no luck.

Regards.