I have heard that most spam filters use a reverse DNS lookup on the originating IP address of the sending SMTP server (or something like that) as part of the verification process. I want to make sure my emails will pass this check. How exactly can I verify that my emails are OK?
In researching it, I've noticed that if I look at the header of a valid email message coming from Gmail, I will see something like this:
Received: from mail-pz0-f185.google.com (mail-pz0-f185.google.com [220.127.116.11]) by mx.google.com with ESMTP id 10si2346996pxi.164.2009.07.22.22.26.19; Wed, 22 Jul 2009 22:26:19 -0700 (PDT)
When I do an nslookup on 18.104.22.168, it responds with mail-pz0-f185.google.com, which is what I would expect. If it responded with a different domain, then I assume that would indicate a problem and it would fail the check.
Is this what the reverse DNS lookup is all about or am I barking up the wrong tree? If I am incorrect in my assumptions, please enlighten me.
EDIT 1: Also, mainly out of curiosity, what exactly would stop someone from putting a false, yet real, IP address and domnain in the header? For example, what if a spammer put the Gmail info in their header? Is it somehow impossible to put a fake IP address in there?
EDIT 2: OK, I think I sort of understand now. It's not so much what's in the header, but whether the connecting IP address can be reversed DNS'ed and then forward DNS'ed back again. Thanks all.