Need to send mail only when the job is not running

I have set up a cron, which will send the mail with not runnnig jobs in the script.sh

15,30,45 * * * * /a/b/c/script.sh | grep not | mail -s "test mail" learner987@gmail.com

But i dont want mail when all the jobs are running, but with the above cron i will get mails without the content(if all the jobs are running).
Please suggest.

man crontab :

Thanks RudiC
but MAILTO is not present in my machine and i dont have permission also.
Is there any other way for it, like by using 'if' command or something.
Can you assist by giving an example.