Nick Christenson of sendmail.com provided the following information. 
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This will work for all versions of Sendmail 8.9 and Sendmail Pro, and all
commercial sendmails based on 8.9.  We believe, but I cannot guarantee that
this includes Solaris 7 and FreeBSD 3.x.


--
LOCAL_RULESETS
# Kludgey Melissa virus checking routine.
# Just need enough of a pattern to match.
# Instructional note:
#       The format for the rule is 
#               RExactly the thing you want to quote
#       No quote marks, no tabs, absolutely nothing in paretheses (like this,
#       they're considered comments and will be removed before they get to
#       the rules).  After the exact thing, then a tab, and the $#error.
#       Note, the $* matches anything, so it's useful for wildcarding.
#       This also scans all messages with Subject: headers and invokes a rule,
#        so there is a performance hit.
HSubject:			$>local_check_header
D{MelissaMessage}"553 Your message may contain the Melissa virus.  Please email postmaster@$j if you have questions."

Slocal_check_header
RImportant Message From $*	$#error $: ${MelissaMessage}
RRe: Important Message From $*	$#error $: ${MelissaMessage}
--