[exim] exim rewrites the "From:" address

When I send an email using: mailx -r , specifying the from address in
the form name@domain, exim rewrites the domain of the From: address and
Sender: address to the first entry in the local_domains list. My server
hosts multiple domains, so I need to be able to specify the domain part
of the email address.

My server uses Debian so the configuration is generated from files
exim4.conf.localmacros (where I set: MAIN_FORCE_SENDER to yes) and
update-exim4.conf.conf (where I set dc_other_hostnames to the list
of domain names on this server). But I checked the generated file
/var/lib/exim4/config.autogenerated and it contains:


.ifndef MAIN_FORCE_SENDER
local_from_check = false
local_sender_retain = true
untrusted_set_sender = *
.endif

which according to my understanding of the documentation should make
exim leave the from address specified in the command
mailx -r name@domain destination
unchanged. But on restarting exim and sending an email with mailx -r,
it doesn't, it rewrites the domain name.

Why is it doing this and how can I stop it?