Postfix multiple ips

Hello 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 example1.com it will be sent email via ip 1.1.1.1 and for example2.com it will sent email via ip 2.2.2.2. The only thing that i am stuck is the hostname, in all the domains i see the hostname is from the hostname server, so n1.example.com. My question is how to customize the hostname as well so it will be different for different domains? If possible how?