Apple released Mountain Lion 10.8.4 update with many new improvements in imessage and many bug fixes this update improve compatibility with many networks  and Microsoft Exchange calendar compatibility improvements . Also fixed many security holes  .

Release notes  :

• Compatibility improvements when connecting to certain enterprise Wi-Fi networks
• Microsoft Exchange compatibility improvements in Calendar
• A fix for an issue that prevented FaceTime calls to non-U.S. phone numbers
• A fix for an issue that may prevent scheduled sleep after using Boot Camp
• Improves VoiceOver compatibility with text in PDF documents
• Includes Safari 6.0.5, which improves stability for some websites with chat features and games
• A fix for an issue that may cause iMessages to display out of order in Messages
• Resolves an issue in which Calendars Birthdays may appear incorrectly in certain time zones
• A fix for an issue that may prevent the desktop background picture from being preserved after restart
• A fix for an issue that may prevent documents from being saved to a server using SMB
• Addresses an issue that may prevent certain files from opening after copied to a volume named “Home”
• A fix for an issue that may prevent changes to files made over NFS from displaying
• Resolves an issue saving files to an Xsan volume from certain applications
• Improves Active Directory log-in performance, especially for cached accounts or when using a .local domain
• Improves OpenDirectory data replication
• Improves 802.1X compatibility with ActiveDirectory networks
• Improves compatibility when using mobile accounts

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