Apple is working on a fix to address issues with the native Mail client in OS X Mavericks



From MacRumors:

There have been widespread reports of Mavericks users experiencing problems receiving messages, sorting messages into folders, and deleting messages. Apple is aware of the problem and has seeded an updated Mail app to both Apple employees and testers in Apple's AppleSeed customer program. Apple is telling testers that the updated Mail app, which carries the same Version 7.0 as the current app but an updated build number of 1822 compared to 1816 for the publicly available version, fixes those issues while addressing overall stability and smart mailbox enhancements.

  Apple Working on Fix for OS X Mavericks Mail App Issues