Edit /etc/mail/sendmail.mc
/etc/mail/sendmail.mc
After the MAILER () definitions, add:
LOCAL_CONFIG Klog syslog HSubject: $>+CheckSubject LOCAL_RULESETS SCheckSubject R$* $: $(log Subject: $1 $) $1
That's a tab after the R$* on the last line, not spaces.
R$*