Email servers for instance need to be on addresses with a reverse DNS pointing to the same name. To fight a spam, otherwise won't be accepted by majority of other email servers.

How can we achieve something like this?

mail44.hamradio.si 44.150.0.10 44.150.0.10 mail44.hamradio.si

That is true, but you do not need to run your mailserver under the hamradio.si domain even when you handle mail for that domain. It would work perfectly fine with a domainname like mail44.s57nk.ampr.org (both forward and reverse) and then in hamradio.si an MX record with that name.

For this kind of thing we use "club callsigns" like pi9noz or pi4nos.

Rob