[exim] Filter behavior when run from shell vs systemd

Hi all,

I'm setting up exim to process emails based on the existence of a set of
centrally managed forward files. So I have an accept router configured as
follows:

email_address_check:
driver = accept
condition = ${if exists{DOMAINS_DIR/$domain/$local_part/forward}}
verify_only

The issue I am facing is that this condition fails when run via systemd,
but passes when run from a root shell. In both cases the command is:

exim -C /etc/exim/exim.conf -bdf -d

This is on a CentOS 7 system with the standard exim package. I have
verified the file and full path are readable by the exim user. I've tried
both exim and root in the systemd EnvironmentFile and both fail the same
way. The unit file looks like this:

[Unit]
Description=Exim Mail Transport Agent
After=network.target
Conflicts=sendmail.service postfix.service

[Service]
PrivateTmp=true
Environment=QUEUE=1h
EnvironmentFile=-/etc/sysconfig/exim
ExecStartPre=-/usr/libexec/exim-gen-cert
ExecStart=/usr/sbin/exim -C /etc/exim/exim.conf -bdf -d

[Install]
WantedBy=multi-user.target

I am at a loss as to what to poke at next. Any ideas what else could be
varying between systemd and a root shell?

Thanks in advance for any guidance or documention references to follow up
on,
-gabe