Reverse DNS Record, PTR, Pointer Record
The reverse DNS record is also known as the PTR record, pointer record or IP resolver. A PTR is a reverse record for an IP address which allows an IP to map to a domain and vice versa. That means you can make hostgator.com point to 10.0.0.1 and make a PTR record so that 10.0.0.1, if checked, would tell you that it belongs to hostgator.com. It lets the receiver of your emails check for possible spoof emails which will be treated as spam. Without this record, the receiver must rely on guessing if your email might be spam.
Generally, when you have a PTR record, your emails have a better chance to deliver to the recipient's inbox instead of the spam folder.
How Mail Servers Check the PTR Record
When the sending server, for instance a VPS, tacocat.hostgator.com, sends an email to Gmail, from the address email@example.com, it will connect to the Gmail server and tell it ,"Hello, I'm tacocat.exampledomain.com."
Gmail then has the option to either say "What email do you have for me?" or "Let me look into this..."
If it's the latter, Gmail will pull an rDNS record on the IP that tacocat.exampledomain.com is connecting from. It does NOT check the domain somedomain.com; that would only occur for reverse user checking which has nothing to do with rDNS records (or if the admin of the mailserver felt especially evil).
If the rDNS record comes up as tacocat.exampledomain.com, then Gmail will be happy.
If the Hostname Does Not Match
In some cases, if the rDNS comes up as somedomain.com and somedomain.com points to that IP address, some email servers will accept that since the mail server IP is able to map back to itself even if its hostname claim does not match. Do not rely on the external mail server being that smart though.
Times when this is not true is when exim is configured to send from other IPs dependent on its hostname; this can be seen by checking /etc/mailips. Most clients do not have this and do not need it.
How Do I Add a PTR Record for My HostGator Account?
Shared and Reseller
HostGator has already set up PTR records for shared and reseller servers by default.