i'm using the yahoo zimbra desktop 9.0, and it works amazingly well.

it's an offline reader for your yahoo account. stores all your emails, in a clone of the website's gui. i just leave this running and never go to mail.yahoo.com again. you can reply, sort mails, delete, mark spam... all the features but offline.

changes synchronize on next check.

brilliant stuff. also has some integration with addressbook, calendar etc, which i don't use.