I'd go step by step.
does it work direct from the cmd line ie
egrep -i 'warn|error' /var/log/messages | mail -s 'issues worth reviewing' some_user@some_host
If that doesn't work, wrapping it in a script and cron isn't going to help.
If the direct cli call doesn't work, do you see any msgs?
Check the logfiles eg /var/log/messages, /var/log/maillog.
Also, try mailq -Ac, mailq -Lq http://www.oreillynet.com/linux/cmd/...p?path=m/mailq
Tell us what distro & version
Which MTA ? eg sendmail, postfix... ? Have a look in /etc.