Try like this,
php /home/USER/public_html/cron/mail.php
check below screenshot once how to set cron job

also pls double check /home/USER is your root directory? if not than cron job will not running. and some of the hosting provider's cron job command is different.
IMPORTANT THINGS TO BE REMEMBER REGARDING CRON JOB
- -first check you are able to run your cron job on every minute, some basic charge of your server, from my thinking you are not able to fired cron on every minute and some of not giving you to fired your cron on every minute.
- if every minute not working than try to run command for 15 or every 30minutes than only you know whether your cron job is running
- first ask to your hosting provider regarding can i able to fired my cron on every minute.
Look Some Example:
//Command to run a PHP5 cron job:
php /home/user_name123/public_html/cron.php
OR
usr/local/php5/bin/php5 /home/user_name123/public_html/cron.php
//Optional flag sometimes required for a PHP cron job:
php -q /home/user_name123/public_html/cron.php
//Command to run a PHP4 cron job:
/usr/local/php4/bin/php /home/user_name123/public_html/cron.php