configure spam expert in cPanel/WHM IP-based Authentication

Navigate to "Exim Configuration Editor" in WHM.

Choose "Advanced Editor". Add the following in "Section: POSTMAILCOUNT" (chaning SMARTHOST to your SMTP hostname)

smarthost_dkim:
driver = manualroute
domains = !+local_domains
require_files = "+/var/cpanel/domain_keys/private/${sender_address_domain}"
# Exclude null sender messages from relaying via the smarthost
condition = ${if or {{!eq{$sender_address}{}} {!eq{$sender_host_address}{}}}} 
transport = remote_smtp_smart_dkim
route_list = $domain SMARTHOST::587

smarthost_regular:
driver = manualroute
domains = !+local_domains
# Exclude null sender messages from relaying via the smarthost
condition = ${if or {{!eq{$sender_address}{}} {!eq{$sender_host_address}{}}}} 
transport = remote_smtp_smart_regular
route_list = $domain SMARTHOST::587

Add the following in "Section: TRANSPORTSTART" :

remote_smtp_smart_dkim:
driver = smtp
hosts_require_tls = *
interface = ${if exists {/etc/mailips}{${lookup{$sender_address_domain}lsearch*{/etc/mailips}{$value}{}}}{}}
helo_data = ${if exists {/etc/mailhelo}{${lookup{$sender_address_domain}lsearch*{/etc/mailhelo}{$value}{$primary_hostname}}}{$primary_hostname}}
dkim_domain = $sender_address_domain
dkim_selector = default
dkim_private_key = "/var/cpanel/domain_keys/private/${dkim_domain}"
dkim_canon = relaxed
headers_add = "${perl{check_mail_permissions_headers}}"

remote_smtp_smart_regular:
driver = smtp
hosts_require_tls = *
interface = ${if exists {/etc/mailips}{${lookup{$sender_address_domain}lsearch*{/etc/mailips}{$value}{}}}{}}
helo_data = ${if exists {/etc/mailhelo}{${lookup{$sender_address_domain}lsearch*{/etc/mailhelo}{$value}{$primary_hostname}}}{$primary_hostname}}
headers_add = "${perl{check_mail_permissions_headers}}"

Save and restart