You might be knowing this solutions already. Here it is just for the record.
To clear Mail-Daemon failure emails from queue on an exim server, you can use this command.
grep -lr ‘Mailer-Daemon’ /var/spool/exim/input/ | sed -e ‘s/^.*\/\([a-zA-Z0-9-]*\)-[DH]$/\1/g’ | xargs exim -Mrm
Tags: exim
