I am confused by your request. For users behind the Peplink, the load balancing will automatically use the external DNS server assigned to the WAN port in use at the time. No further action is required.
If you have a server on your internal LAN, and you want your users to access the server via a URL name, you can enter the name and URL on Network > LAN settings > DNS Proxy > Local DNS records. Of course that entire box has to be enabled.
It sounds like you are seeking advice on how to edit a registrar like GoDaddy for use outside your LAN. On your registrar’s DNS settings (not in your Peplink router), you will need to create an A-record for mail.xx.com, which points to the numeric external IP address you plan to use for your mail server. That is typically a unique IP used only for the mail server, not the same one you would use to externally manage your router.
On the registrar you also need to create an MX record with a host of @ (the at sign), which points to mail.xx.com.
With the registrar changes made, within an hour users outside your LAN can type mail.xx.com into their browser and be pointed to the external IP address of your Peplink router. At that point the router needs port mapping in order to connect the request to a mail server sitting on your LAN. It may also need a change in your inbound firewall depending on your current firewall settings.