Mac OS X v10.6: "You can’t use this version of Mail…" alert after installing Security Update 2012-004

After installing Security Update 2012-004, you may not be able to open Mail. This alert may appear: "You can't use this version of Mail with this version of Mac OS X. You tried to open Mail version 4.5 (1084/1085). It can’t be used on this version of Mac OS X. Try opening the version of Mail located in the Applications folder on your computer."