There he convingty analyzes that Yahoo IMAP is to blame and that it behaves badly on other platforms like iPhone too (although the access data returned isn't that noticable there as on WP7).
To sum it up: WP7 works according to IMAP specification and Yahoo-IMAP returns far to much data in return and the extend on how noticable this is depends on the way the mail client queries the server (but that doesn't mean a different way is better - it's just that for the buggy Yahoo implementation they are more lucky).