Apple today released Mail Update 1.0, which resolves an issue in which some users experienced crashes of Apple's Mail application after upgrading to OS X 10.5.6. The update weighs in at 58 MB and requires OS X 10.5.6.
This update addresses stability issues with Mail. After updating to Mac OS X 10.5.6, Mail may unexpectedly quit. This can be caused by using a copy of Mail that wasnt updated properly (you may need to install the Mail Update).
Apple had previously posted a tech note regarding this issue offering several suggested fixes, including reinstalling the OS X 10.5.6 update and removing or updating third-party Mail plug-ins. This issue is in addition to Apple's acknowledgement of installation difficulties being experienced by some users attempting to upgrade to OS X 10.5.6.