Linux boxes like to be able to send mail to notify you of things like failed cron jobs. However, setting up a full-blown mail transfer agent on all of your linux boxen is a bit overkill if all you want is for system mail to be sent to, say, a gmail account. For that, you can use something simple like ssmtp.
Under debian, install ssmtp like so:
apt-get install ssmtp
then edit /etc/ssmtp/ssmtp.conf to look like so:
root=USERNAME@gmail.com mailhub=smtp.gmail.com:587 UseSTARTTLS=yes UseTLS=yes AuthUser=USERNAME@gmail.com AuthPass=PASSWORD FromLineOverride=NO
(replace USERNAME and PASSWORD with your gmail account info).
test it out by creating a cron entry. run 'crontab -e' and put this in your crontab:
* * * * * echo foo
wait a minute or two and an email should show up in your gmail account.
once you've verified that it works, run 'crontab -e' again and delete the cron entry.