Apple's Mac operating system has a reputation for being stable and having no bugs. Even Apple has occasional glitches though, just like Microsoft. Apple has admitted that after the recent 10.5.6 update for OS X there was a problem with Mail.
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A bulletin on Apple's support site says: "After updating to Mac OS X 10.5.6, Mail may unexpectedly quit. This can be caused by having outdated third-party Mail plug-in installed, or by using a copy of Mail that wasn't updated properly (you may need to install Mail Update 1.0)."
Two third party plug-ins known to cause issues with Mail after the new update are OMIC and GPGMail. If removing the plug-in doesn't do the trick, Apple says go back to 10.5 and try the update again.
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