/usr/share/sendmail/cf/feature/badmx.m4 is in sendmail-cf 8.15.2-8.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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#
# Copyright (c) 2006 Proofpoint, Inc. and its suppliers.
# All rights reserved.
#
# By using this file, you agree to the terms and conditions set
# forth in the LICENSE file which can be found at the top level of
# the sendmail distribution.
#
#
divert(0)
VERSIONID(`$Id: badmx.m4,v 1.2 2013-11-22 20:51:11 ca Exp $')
divert(-1)
define(`_BADMX_CHK_', 1)
LOCAL_CONFIG
Kmxlist bestmx -z: -T<TEMP>
Kbadmx regex -a<BADMX> ^(([0-9]{1,3}\.){3}[0-9]){0,1}\.$
KdnsA dns -R A -a. -T<TEMP>
KBadMXIP regex -a<BADMXIP> ifelse(defn(`_ARG_'), `', `^(127\.|10\.|0\.0\.0\.0)', `_ARG_')
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