AartJansen: So RDNS is like NSlookup ?
The "principals" are the same - more like DNS (in reverse).
Where DNS will resolve a name to an IP, RDNS will resolve an IP to a name. RDNS is frequently used by many systems to verify the source of the Email by "double checking" with RDNS that the Email sent was actually coming from the Domain.
NSLookup is a command used to place a query against a DNS Server.
Google "reverse dns lookup" and use one of the many free services to see the results.
For example (IP has been changed to protect our our customer's identity):
18.104.22.168 resolves to "210-99-99-199.adsl.xtra.co.nz"
Top Level Domain: "co.nz
When you get your ISP to create a RDNS record for you system, the result should be this instead:
22.214.171.124 resolves to "210-99-99-199.mybusiness.co.nz"Top Level Domain: "co.nz
This way, a receiving Email system can verify that it is coming from the actual Mail Server that sent the Email and not the Blacklisted system.