The issues with the OS X Mavericks Mail app are well documented, and we've heard talk of an update heading our way. What we didn't know is that registered Mac developers can already download an update. It actually became available on October 31 and went unnoticed – no seeding emails seem to have been sent out. So, if you're a registered developer, go get it!
One of the major issues is the seeming lack of proper compatibility with Gmail, so we're hoping that it delivers the proper fixes for that. And better still, if it's in the developer portal, it means it's real, and it should be seeing a consumer release in the not too distant future.