Emails from local applications on another server

Hi there
I have added an additional domain to my box and I can email from and to quite happily. The only issue I have is that the additional domains website is hosted on another (digital ocean) vps. I am not able to receive service emails and website form submissions from that server to my box. I should add I originally had my postfix/dovecot email server running on the other server. I can’t seem to get work out the right settings to make this work.

Your help would be appreciated.