The crontab files are kept in /var/spool/cron/crontabs. There is one file for each user who has asked for commands to be run by cron. However you should not edit the files directly, but only as described below.
Since the mail collection must be run by root, you should become superuser before you try to set up your crontab entry.
This is how to do it:
# crontab -e(This invokes vi; you can then type in the required entry at the bottom of the existing crontab file. When you write the file away, the crontab entry will be updated.)
This is the entry you need to put in (with appropriate amendments for your own requirements):
# Collect mail from ISP 01 07-17,20,22 * * * /usr/local/etc/poll.mailThe first five space-separated fields in the line specify the time at which the command is to be run. The rest of the line is the command itself.
The time fields are, in order: