Change mail server name and not hostname

Hello, I am on freshly installed centos 7 with latest iredmail and everthung works. The how to in install said and I added(the entries are made up and ip's also is made up):

# Part of file: /etc/hosts
127.0.0.1   main.mokoko.com main localhost localhost.localdomain

Now, I want my mail server to be: mail.mokoko.com but I do not want to change hostname and fqdn. Should I just change it in postfix main.cf to mail.mokoko.com and add dns record?

Also, shouldn't' the entrie in /etc/hosts rather be like  this after the installation:

# Part of file: /etc/hosts
127.0.0.1    localhost localhost.localdomain
34.23.122.288 main.mokoko.com main (ip is fake just for example)
?

Thank you