Setting up different mail server for one domain in zpanel

I have two domains in zpanel server:

abc.com
def.com

There are many email accounts which belong to these domains. In this example I am using

mark@abc.com
john@def.com

When I use same local mail server and send email from one email to another is not a problem. But when I change mail server of def.com to another server for example to microsoft office 365 then I can't send email from mark@abc.com to john@def.com.

Below is the error I am getting:

SMTP Error (450): Failed to add recipient "john@def.com" (4.1.1 <john@def.com>: Recipient address rejected: User unknown in virtual mailbox table).

How can I solve this problem? I think zpanel is looking for local server to deliver mail.

I tried following:

created /etc/postfix/transport and inserted following:
def.com smtp:mailserver.com

$ postmap /etc/postfix/transport
then uncommented transport_maps = hash:/etc/postfix/transport in /etc/postfix/main.cf and at last restarted postfix.