In WHM > Exim Configuration Editor > Advanced Editor, put the following in the topmost box:

domainlist blocked_domains = lsearch;/etc/blockeddomains

Locate the \"ROUTERS CONFIGURATION\" section, and right below these lines:

==============
democheck:
driver = redirect
require_files = \"+/etc/demouids\"
condition = \"${if eq {${lookup {$originator_uid} lsearch {/etc/demouids} {$value}}}{}{false}{true}}\"
allow_fail
data = :fail: demo accounts are not permitted to relay email
==============

Put the following lines:

reject_domains:

driver = redirect
# RBL Blacklist incoming hosts
domains = +blocked_domains
allow_fail
data = :fail: Connection rejected: SPAM source $domain is manually blacklisted.