You can use sendmail directly to send the email.
Using custom shell script would make it "cleaner".
1 make crontab execute custom script
@reboot .../reboot-notify
2 make the script use sendmail directly
#!/bin/sh
HOSTNAME=`hostname`
/usr/sbin/sendmail -i [email protected] <<END
Subject: System restart
Server has restarted: $HOSTNAME
END