You have new mail.
Sometimes you forget to give that cron task somewhere to output its results... so it dumps them in your mail box... 📬😣 all 30,000 times it failed to run.
You don't have to delete all those emails one by one 😉
echo 'd *' | mail -N
d * means delete all mail. The
-N Inhibit the initial display of message headers when reading mail or editing a mail folder.