Mac OS X v10.6: "You can't use this version of Mail…" alert after installing Security Update 2012-004:
http://support.apple.com/kb/TS4424?viewlocale=en_US&locale=en_US
Fellow user Grant Bennet-Alder offers this solution:
Some users have reported this problem if the Mail Application has been moved out of the top-level /Applications folder, or duplicated in another location.
When the Security Update is done, the old version of Mail is disabled.
The solution has been to:
1) make certain Mail is in the /Applications folder
2) There is no other copy anywhere else.
3) Once steps 1 and 2 have been done, Manually download and re-apply the Security Update (2012-004) by hand.
Security Update 2012-004 (Snow Leopard)
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If the Mail.app has been LOST, it can be re-installed by applying the 10.6.8 version 1.1 combo update. But this update is quite large and it is usually not necessary:
Mac OS X 10.6.8 Update Combo v1.1