Get Sendmail to Log the Subject Line of an E-mail

Edit /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.