[Mashiro]

For my experience dead.letter also appeared when I accidentally deleted a user account on a VPS that was meant to do backups. At this point I got dead.letters every day with exactly by root to root because it was a system e-mail mailing itself to tell there is an issue somewhere.

I didn't even know that the user account was made for backups. So the other server manager recreated it and dead.letter stopped appearing. It can have many reasons. Generally though it's something related to mailing.