/etc/mail/peers/provider is in sendmail-base 8.14.4-4+deb7u1.
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 | #/etc/mail/peers/provider
#
# Sample provider file for sendmail
# used to provide ISP information (masquerade_as, smarthost, etc.)
#
# This is what I use when connecting to mindspring.com:
# remove the leading dnl to activate
LOCAL_CONFIG
dnl define(`SMART_HOST', `mail.mindspring.com.')dnl
dnl MASQUERADE_AS(mindspring.com)dnl
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