Imagine you are seeing a lot of Spam mails originating from a domain say 163.com in your server logs. We can blacklist the domain 163.com from sending mails through Exim mail server.

Create a file named /etc/eximblacklist and add the entry "163.com" (without quotes). That is, add the domains you need to blacklist one per line.

Now go to WHM >> Exim configuration editor >> Advanced Editor.

Add the below lines in the first section just below the line "#!!# cPanel Exim 4 Config"

domainlist exim_blacklist = lsearch;/etc/eximblacklist

Now add inside the section under "ROUTERS CONFIGURATION "

reject_domains:

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

Save the configuration and see the exim error log. You can see the domain blocked.