This file is indexed.

/etc/mail/service.switch-nodns is in sendmail-base 8.14.4-8+deb8u2.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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# /etc/mail/nsswitch.conf
#
# This is the format that sendmail expects (no colon after the service)
# This file will prevent dns usage
# 
passwd		files
hosts		files
aliases		files