Hi,
Seems like Mail (or some other program) then is accessing it. That's different than on my MBP (OS X 10.8.1).
Are you rinning OS X 10.8.x (Mountain Lion)?
Do you also have a "com.apple.mail.plist" file in the ~/Library/Containers/com.apple.mail/Date/Library/Preferences/ folder (sorry: the forum margins seem to gobble the loooong path+file name)?
If yes, does that also get accessed/modified?
If your answers are "yes" to all of the above questions, I'd guess that this is what Mail really needs. Then, I'd recommend (after quitting Mail.app): duplicate the /Library/Preferences/com.apple.mail.plist file for backup (just in case, but change the name so you remember it's a backup), then delete /Library/Preferences/com.apple.mail.plist, and re-launch Mail.app.
Hopefully, the sandboxd error messages will stop. You can now go back and delete the backup file you've left in the /Library/Preferences/ folder.🙂
If your answers were not all "yes" to the above questions, your Mail.app probably really does need that /Library/Preferences/com.apple.mail.plist file (and it has accessed/modified it recently). You can still delete it, but may well have to re-configure all of Mail.app preferences (including the various e-mail accounts) from scratch.😟 But, the sandboxd problem should be gone...
Cheers, Tristan