/etc/mailto.conf is in mailto 1.3.2-3.
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 | # This is a template of a mailto.conf file
#
# Entries look like the following: email and optional realname of
# permitted receipant.
#
# joey@debian.org (Martin Schulze)
root
|