Apple has released OS X 10.9.1. The update is the first major update for OS X 10.9 Mavericks since it shipped in October. It includes a number of fixes for mail issues in Mavericks, including improved support for Gmail in OS X's Mail app, Smart Mailbox improvements and improvements to group contacts in Mail.
OS X 10.9.1 also resolves a VoiceOver issue with spoken sentences containing emoji and now updates shared links periodically when open in Safari's sidebar.
You can grab the OS X 10.9.1 update by running Software Update on your Mac.