/etc/exim4/conf.d/router/mmm_mail4root is in exim4-config 4.82-3ubuntu2.
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 | ### router/mmm_mail4root
#################################
# deliver mail addressed to root to /var/mail/mail as user mail:mail
# if it was not redirected in /etc/aliases or by other means
# Exim cannot deliver as root since 4.24 (FIXED_NEVER_USERS)
mail4root:
debug_print = "R: mail4root for $local_part@$domain"
driver = redirect
domains = +local_domains
data = /var/mail/mail
file_transport = address_file
local_parts = root
user = mail
group = mail
|