Is there a way for me to limit the EXIM allowed relay hosts to localhost and one other IP address? I'm finding the relay_from_hosts option in exim.conf - but I assume that would be over-written upon updates. So in WHM can I set that?
Would the "Trusted SMTP IP addresses" option in "WHM >> Exim Configuration Manager >> Basic Editor" server the purpose you are seeking? Per it's description:
IP addresses exempt from all SMTP sender, recipient, spam, and relaying checks. IP addresses you enter here are stored in /etc/skipsmtpcheckhosts. These senders must still use an RFC-compliant HELO name if the Require RFC-compliant HELO setting is enabled.