cPanel servers use a service called antirelayd to force users to authenticate by either specifying an SMTP password or using pop before SMTP. Any IP that has been authenticated is automatically added to /etc/relayhosts, but is then removed by antirelayd 30 minutes later. If you want to permanently allow an IP to relay mail through your server, you have to add the IP to /etc/alwaysrelay.

This file will usually not exist by default. All you need to do is create it, then list your IP’s line by line:

/etc/alwaysrelay
192.168.1.2
192.168.1.3