Can't receive emails from certain addresses

Can't receive emails from certain addresses

Hi team, I am doing some work for a client and we have just about got everything sorted regarding Zoho mail, but one quandary still exists. The domain of the client is nwctravel.com.au. Emailing @nwctravel.com.au from every other domain is okay except knightsmith.com.au. All other email is fine. Any ideas? Thanks team.