As long as 10.8 Mail app is running - every 30 seconds, I get this error, and I have no idea how to fix it....
Seems like something that should be addressed.
7/26/12 12:54:52.037 AM sandboxd: () Mail(2271) deny file-read-data /Library/Preferences/com.apple.mail.plist
I was seeing this too. In my case it was reporting the error every 2-3 seconds.
I notice that my "/Library/Preferences/com.apple.mail.plist" was quite old, and clearly came from a previous version of Mail (in my case, perhaps from Mac OS X 10.5 even...)
Is everyone seeing this done an upgrade installation of Mountain Lion?
I renamed the file and the errors stop immediately. I had a look in it, and it doesn't seem to have anything important for current Mail, so I've actually deleted the file. No more errors.