I have a couple of filters which move selected emails from Inbox to specific folders. When this happens the inbox still contains the original messages, although the message count is reduced or zero, the folder size reflects the remaining text, and compacting the folder does nothing. If I then receive a further , unfiltered message and either delete it or move it into a permanent folder, compressing the Inbox then works. I tried using Properties->repair on the Inbox when it has these remnants and it restores the filtered messages, which can be deleted as I already have the filtered copies. I suggest there is a bug in filtering which leaves the Inbox in an invalid state. As I like to keep my Inbox clean I find this behaviour annoying. I think this should be classed as a bug.