While you are deleting some files which fill up your disk you may hit -bash: /bin/rm: Argument list too long error message.
So you can use below syntax to can delete those files:
# find . -name "DIR_PATH/*" -type f -exec rm -f {} \;
or:
# find DIR_PATH -name "*" -type f -exec rm -f {} \;
You can also delete files with a specific extension (ex.: .php):
# find DIR_PATH -name "*.php" -type f -exec rm -f {} \;
or:
# find . -name "DIR_PATH/*.php" -type f -exec rm -f {} \;
PS: In our case DIR_PATH=/var/spool/postfix/maildrop (there are like a half million files in that directory).