You don't need to be an ISP to change reverse DNS mappings, you can ask your ISP (or do this via UI provided by the ISP). Most cloud providers allows to configure reverse name, the same for DCs which offer servers for rent or provide co-location. Broadband ISPs offer this for business customers (unless they face no competition and get away with being customer unfriendly). Though it is not a good idea to host a mail server on a broadband connections - they usually blacklisted because of history of spam.
> And prevents you running multiple domain mail server on single ip
No it is not. You have to have a pair of mappings: IP -> hostname -> IP, but practically nobody checks that domain of hostname matches domains used in emails. Most mail providers send mail with multiple domains in mail form using shared pool of servers which do not use customer's domains in their hostnames.
First part is repeating myself (Where did I say you need to be isp?!! Injecting something false and then arguing about it...), second part google does check reverse dns.
I am running self hosted mail server on broadband static ip for close to 20 years and until the reverse resolve was done I was categorically thrown into spam on gmail. DKIM or not. Once the reverse DSN was set it stopped (after a while)...
Well just google it you have enough of showcases there.
> And prevents you running multiple domain mail server on single ip
No it is not. You have to have a pair of mappings: IP -> hostname -> IP, but practically nobody checks that domain of hostname matches domains used in emails. Most mail providers send mail with multiple domains in mail form using shared pool of servers which do not use customer's domains in their hostnames.