Exchange Server Email Issue

Hi All, I am an IT engineer from Papua New Guinea, I am engaged by my client to assist with Email hosting currently done by a local ISP who happens to be using C Panel. Currently my client has Exchange Server 2016 in its LAN and using MX for external email routing. We have an issue with receiving external email. External mail destined for the exchange server 2016 mailbox sits on the C Panel login mail account and is not forwarded to the mailbox in Exchange Server 2016. If you could assist. Thank you Richard.