According to people familiar with the forthcoming update, Apple made the Mail Update for Mavericks available to developers outside of the AppleSeed community for further testing, the next step toward a public release.
Apple is asking developers to focus on three main areas with the latest build: Mail compatibility with Gmail, Mail smart mailboxes and Mail stability. A number of users have complained of issues with Mail after upgrading to OS X 10.9 Mavericks, though the company has been mum on the upcoming fix. More specifically, Apple clarifies that the update "improves stability and compatibility with Gmail, and includes fixes for users with custom Gmail settings."
The Mail update was first issued to AppleSeed members last week. At the time, sources said that along with Gmail compatibility, problems with Microsoft Exchange were also mentioned as an area of concern.
Users have taken to Apple's Support Communities forum to air their concerns about Mavericks Mail's many issues, with one of the largest threads, pertaining to Gmail folder behavior, now at over 20,500 views.
As with the AppleSeed version, Apple has not specified when the update will be made available for public consumption.