Basic Questions about Reverse DNS

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From: "TC" <golemdanube@xxxxxxxxx>
Date: 27 Nov 2006 18:12:22 -0800

I have a couple of basic questions about "Reverse DNS":

1. How does Reverse DNS deal with multiple hostnames per IP address?

Every example I've seen of Reverse DNS shows only one hostname per IP
address. Isn't it common, however, to have multiple hostnames per IP
address? How does Reverse DNS deal with that?

In my specific case, I use one mail server to handle three domains.
I've created three different PTR records for the server's IP address,
but I don't think it's working.

2. When a mail server uses Reverse DNS to identify spam, what exactly
is it doing? Is it looking at the return address of the email, and
making sure that the IP address and hostname match both forward and
backward? (Somehow, I don't think that's right, but I don't know what
is...)

I've discovered that some (but not all) of my emails are failing a
Reverse DNS spam check, but I can't figure out why.

All help is appreciated.
-TC

.

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