reverse dns what exactly it is ? can anyone explain me ?

cheers

It's the 'name' assigned to an IP so if someone does a lookup on an IP, this 1 name will come up. By default, it's assigned to the datacenter which is how we keep it so the IP is anonymous and our resellers can use the same IP without their customers tracing it back to us. But some people have this renamed so it traces back to the resellers name, in which case, they would need a different IP for each reseller. There may be some problems with some providers such as Yahoo, AOL, Hotmail, etc not accepting mail from an IP that doesnt have rDNS, but as long as the IP is assigned SOME kind of name, most mail should go through and SM is pretty good about assigning the IP a default name before you get the IP.