Mail Update 1.0 addresses a problem where a copy of Mail that wasn't properly updated while installing the 10.5.6 update unexpectedly quits. For those who have installed 10.5.6 and whose copy of Mail doesn't show a version number of 3.5 (930.3), the update should show up in OS X's Software Update mechanism.
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If you have the proper version number and still are experiencing crashes, Apple suggests updating or removing third-party plug-ins, which may be causing the crashing problems.
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