The latest Mavericks beta build 13C48 comes one weeks after Apple released the third beta version of the upcoming OS X 10.9.2 maintenance update to developers in mid-January.
According to the accompanying release notes, Apple is still focusing on Mail and Messages, two built-in features that have caused problems for many users since Mavericks first launched in October. Those affected have complained of Gmail compatibility issues and erratic Smart Mailbox behavior.
In addition to Mail and Messages, Apple requests developers test server message block (SMB2), graphics drivers, VoiceOver and VPN services. Apple switched from AFP file sharing to SMB2 with Mavericks in an effort to enhance performance, security and cross-platform compatibility.
Developers can download the new build via Software Update or Apple's developer Web portal.