If you deleted it and the space is still not freed, then the process has a file descriptor open on it.
Try to find the PID of the process using, for example:
for process in /proc/[0-9]*; do for fd in $process/fd/*; do file=$(readlink -f $fd) if [ "$file" = "/root/dead.letter" ]; then echo $process fi done done
Then kill him / them.
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