can't delete folders - insufficient permission

As of this evening, I am no longer able to delete folders/subfolders in my inbox.  I get the message :

"Warning: Mail folder "INBOX.Trash.Cast" could not be created (maybe due to insufficient permission on parent folder INBOX.Trash or due to an invalid folder name)"

I have tried this with several folders, but not all, and receive the same response.

I'm using Outlook 2010, which delivers a similar message if I try to delete from there.

Thanks in advance.