Postfix multiple ips

I have a vps with multiple ips and multiple domains, every domain has a dedicated and unique ip, i assigned via posftix as well this schema, so for it will be sent email via ip and for it will sent email via ip
The only thing that i am stuck is the hostname, in all the domains i see the hostname is from the hostname server, so
My question is how to customize the hostname as well so it will be different for different domains?
If possible how?