Apple released the latest update to OS X Mavericks today... 10.9.1. This update focuses on Mail improvements (specifically with better Gmail access). Improvements were also made to Smart Mailboxes and Mail search, while an issue that prevented contact groups from working properly has also been addressed.
As expected, the update is free, so there's no reason not to download it today. To download and install the update, go to Apple > Software Update... on the menu bar.
For more information, visit http://support.apple.com/kb/HT6065.