If you have just updated Snow Leopard:
Mac OS X v10.6: "You can't use this version of Mail…" alert after installing Security Update 2012-004:
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.
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: