Here we discuss a particular aspect of how the Internet is administered and managed that creates a problem for an individual wanting to manage their own email server. If you have understood what was discussed above, you realize that while human beings tend to think in terms of domain names, computers and devices communicate using IP addresses. In most situations, this is not a serious problem because whomever is assigned authority over a domain name, usually for paying a yearly fee, usually also acquires the authority to map that domain name to a particular IP address (using a DNS Zone A record).
That part of the system does not present a problem for a single individual setting up a server, because the person or entity leasing the domain name usually also has the authority to map it to a particular IP address.
Unfortunately, the authority that administers IP addresses is different and traditionally things are handled differently. If you are a large organization or commercial establishment you can lease a block of IP addresses (for a substantial fee) and acquire the rights to administer them. Internet service providers (ISPs) that lease these IP addresses assign them to their customers when they connect to the Internet. But the customer themself usually has no authority over the IP address's Zone Record. It is a PTR record in the IP address's Zone Record, which is stored under the arpa tld zone, that determines the result of a reverse DNS lookup, also called an rDNS lookup. Most ISPs, when they lease IP addresses, set the PTR addresses to point to some variation of dynamic-WW-XX-YY-ZZ.isp-name.net, where WW.XX.YY.ZZ is the ip address.
The above handling of PTR records for IP addresses is normally not a problem except that some telecommunication companies, probably in violation of numerous RFCs, set up their email relays to not forward email from servers where the rDNS lookup does not point to the server's domain name. I am skeptical of their claim that this reduces spam.