/usr/share/sendmail/examples/db/userdb 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 9 10 | #User names and the inbound(maildrop)/outbound(mailname) addresses
#dest machine/domain names must also be in sendmail.cG for masquerading
# NOTE: do NOT use FEATURE(stickyhost) with userdb
# NOTE: be carefull with local delivery, defeats plussed users
#
# NOTE: Combined with virtusertable and/or aliases, Genericstable
# can do everything the userdb can and more (and the difference
# is growing bigger). The userdb is basically obsolete.
#:default:mailname badlands.org
#cowboy:mailname cowboy@vnet.ibm.com
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