Fortigate- Zone creation query

Hi everyone,

let me explain first of all what I am doing while creating a zone in the FortiGate firewall for example -

after creating zone I just wanted to only mail server can go to outside I mean towards the internet rest of the servers can not go outside.

but when I am going to create a policy then there only shows the server zone in which zone I have added all the servers.

Is there any way to create the policy If we have one zone in which all the servers are added?