As you already know how to set on up I'd suggest you do that for your current server, if you haven't already. If you have then I'd also suggest you check to see if it's correct.
My impression was they were trying to tell me how to configure something other than what I was already working on. Thus, my question. Your reply, however, merely told me to go use Yahoo. That's not helpful when I'm asking for step-by-step instructions for setting up rDNS in Zimbra.mail server without reverse DNS configured