/etc/exim4/passwd.client is in exim4-config 4.89-2+deb9u3.
This file is owned by root:root, with mode 0o640.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | # password file used when the local exim is authenticating to a remote
# host as a client.
#
# see exim4_passwd_client(5) for more documentation
#
# Example:
### target.mail.server.example:login:password
|