/usr/share/sendmail/examples/db/domaintable is in sendmail-base 8.15.2-3.
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 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | # /etc/mail/domaintable
# Copyright (c) 1998, Richard Nelson <cowboy@debian.org>.
# Time-stamp: <1998/10/27 10:00:00 cowboy>
# GPL'd config file, please feed any gripes, suggestions, etc. to me
#
# Function:
#    Specify a mapping from old host name to new host name.
#    Commonly used to support old (or merged) domain names.
#    The host name in the lhs will be rewritten to that in the rhs
#
# Usage:
#    FEATURE(domaintable[, type [-o] /etc/mail/domaintable])dnl
#    makemap hash domaintable < domaintable
#
# Format:
#    old fqdn		new fqdn
#
# Examples:
#    debian.com		debian.org
#    .debian.com	%0.debian.org
#
# Notes:
#
# Related:
#
#.cv.lexington.ibm.com  %0.lexington.ibm.com
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