The right FQDN for the server would be servername.heavy-water.com
You can choose any name for servername as long you don’t intend to use that name as a subdomain.
You have to distinguish between domain as Samba4 domain and domain you registered at your registrar. They are 2 completely different entities. The only thing when they come together is when you use the registered domain name as (part of) the Samba4 domain.
Is it a problem to give the NS a other FQDN that is not registered in public DNS when i set
the HELO with the command config setprop postfix HeloHost to mail.heavy-water.com?