de acordo com a página man pode desabilitar o cron mail adicionando -m off
ao daemon args
-m This option allows you to specify a shell command to use for sending Cron mail output instead of using sendmail(8) This command must accept a fully formatted mail message (with headers) on stan‐ dard input and send it as a mail message to the recipients specified in the mail headers. Specify‐ ing the string off (i.e., crond -m off) will disable the sending of mail.