reverse is dns resolution on the ips isp hosting side.
your provider who own your ip address
assigns your domains mx record to your ip in a ptr text record on the dns server on there network. this allow you email to be verified and confirms you are a legitimate domain, not trying to hide or mask yourself.
your mx mail domain should always match your reverse ip resolution.

how to setup a domain on the a domain registrar -with out waiting for Dns Updates.
purchase domain at a domain registrar like cheap domains or godaddy
locate your ip address for your webserver in your welcome email
login to registrar add ips or edit host records
change ip address to your above ip for the following
-*
-www
-@
create a record for mail
create a txt record for spf entry
v=spf1 ip4:x.x.x.x/32 ~all"
creat mx record...