Have you ever tried to type “mail” at comment prompt? Command line tool mail uses for reading system mail messages on your system. However if your system is running as a webserver than user account www-data probably has got many messages. If you want to remove all of the mails for user www-data just go to:
cd /var/mail/
ls -l
Make sure that file www-data is listed.
Now you can safely remove that file by typing:
rm www-data