/etc/mail/service.switch-nodns is in sendmail-base 8.15.2-8.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | # /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
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