I can access another inbox with no trouble. I need to access a sub-folder of this inbox - I have permission for this but cannot find how to access. Is it possible?
The owner of the mailbox needs to assign the appropriate permissions at the
folder level, and grant you at least "reviewer" permissions at the root
mailbox folder level.
Then you'll need to add their mailbox to your Exchange server service
properties so it will open in your folder list - it's in the advanced tab of
the Exchange server properties.