This is not really a ccleaner issue. I have notified postbox support. I have tested and with postbox client open ccleaner hangs at 1 percent complete. It will hang for hours if you let it. When you close postbox, ccleaner completes cleaning
Hi kevmcm.
If you can see in the scanning window which postbox location CCleaner is hanging on (file, folder?), then until you get some kind of resolution, you could add that location (or the entire postbox folder) to CCleaners "Exclude" list to save you closing it down ...
http://www.piriform.com/docs/ccleaner/using-ccleaner/excluding-files-folders-and-registry-entries
... assuming you haven't done that already.