In the WHM, go to : Exim Configuration Editor, Advanced Editor

In the Router Section Add the 4 lines  (Look at the screen shot)

static_route:
driver = manualroute
transport = remote_smtp
route_data = ${lookup{$domain}lsearch{/etc/staticroutes}}

After go into SSH and edit the following file :

nano /etc/staticroutes

The structure will be : (domainname: ip)

domain.com: 222.222.222.222

To ensure Exim will accept the domain to relay edit the following file :

nano /etc/secondarymx

The structure will be : (domainname)

domain.com

On the new Release of CPANEL, the screen shot is :