According to people who have seen OS X 10.9.3 beta build 13D12, Apple is asking developers to focus on Graphics Drivers and Audio. Notably absent from the release notes is Mail, which has been a thorn in OS X Mavericks' side since it debuted in October of 2013.
Despite a standalone patch and continued work through maintenance updates 10.9.1 and 10.9.2, users are still complaining of erratic Mail app behaviors including issues with Smart Mailbox functionality and problems with Gmail.
Apple latest released OS X 10.9.2 last week with a fix for the "goto fail" SSL security flaw and feature additions like FaceTime Audio and iMessage blocking.
Developers can download the latest OS X 10.9.3 beta via Software Update or Apple's developer portal.