This can happen if the Mail app was moved from Applications to another location, such as the desktop, before installing the Security Update.
- Hold down the Option (⌥) key and choose Apple () menu > System Profiler… to open System Profiler.
- Click "Applications" in the "Software" section. Note: The spinning gear in the top middle indicates a search in progress. Once completed, sort by application name and look for "Mail" to see all copies.
- Make sure Mail is located in the Applications folder. If it is not, drag it back there and enter an administrator name and password if prompted. Tip: Choose Go > Applications in the Finder to easily open the Applications folder.
- After moving Mail back to Applications, follow these steps