I really wanted to display a progress bar, but I couldn't find an easy way to do it without requiring a plug-in.
[robg adds: To use this script, paste the text into Script Editor, then save it to your user's Library -> Scripts -> Mail folder. I haven't tested this one (my Mail is already sorted into numerous folders based on content), so I'm not sure if it's 10.4 only or not. Note that the script will move messages by default; if you'd like to just delete old mail instead, remove the DestinFolderName entry on the second line (i.e. set DestinFolderName to "").]

