From Computer Tyme Support Wiki

DNS Lookup to Separate the domain part of a hostname

This DNS lookup helps you find the main domain part of a hostname. Sometimes it is called two level TLDs and three level TLDs. Lookups are accomplished through DNS calls as follows:

dig perkel.com.rb.junkemailfilter.com   - returns 127.0.0.1
dig perkel.co.uk.rb.junkemailfilter.com - returns 127.0.0.2

This is a service of Junk Email Filter

Exim Configuration

# Separates the domain part of a hostname - ftp.perkel.com returns perkel.com
# DNS lookup returns 127.0.0.1 for single level domains
# DNS lookup returns 127.0.0.2 for two level domains
# DNS lookup returns 127.0.0.3 for three level domains

warn	!condition = ${if eq{$sender_host_name}{}}
	dnslists = rb.junkemailfilter.com/$sender_host_name
	set acl_c_rb = $dnslist_value

warn	condition = ${if eq{$acl_c_rb}{127.0.0.1}}
	set acl_c_sender_host_domain = ${sg{$sender_host_name}{\N^.*\.(.*\..*)$\N}{\$1}}
	add_header = X-Sender-Domain: $acl_c_sender_host_domain

warn	condition = ${if eq{$acl_c_rb}{127.0.0.2}}
	set acl_c_sender_host_domain = ${sg{$sender_host_name}{\N^.*\.(.*\..*\..*)$\N}{\$1}}
	add_header = X-Sender-Domain: $acl_c_sender_host_domain

warn	condition = ${if eq{$acl_c_rb}{127.0.0.3}}
	set acl_c_sender_host_domain = ${sg{$sender_host_name}{\N^.*\.(.*\..*\..*\..*)$\N}{\$1}}
	add_header = X-Sender-Domain: $acl_c_sender_host_domain