Yes, it is possible to have a "legitimate" mail server set up on an ADSL line with a subscriber network RDNS. It is also theoretically possible to host a "legitimate" mail server on a dial-up connection, on an open proxy, on a server with an open relay or on an IP range in China that has been blacklisted by every single DNSRBL in the universe. There are all kinds of remote possibilities. I draw the line when 99.9%+ of the traffic is spam.
When I last checked, a test showed that about 10%-15% of legitimate E-mail was coming from IPs that would have been blocked by a "subscriber network" test. That was perhaps a year or two ago, however, so the numbers may have changed.
At least with Imgate the odd legitimate sender receives notice of the delivery failure, unlike with Scott's weighting system.
But with "Scott's weighting system", the E-mail goes through.
So you're comparing bouncing a legitimate E-mail with it being delivered. I know which I would pick.
Anyone who goes the cheap route and routes all of their e-mail via a single MX on an ADSL connection without bothering to configure RDNS is truly getting what they pay for.
It isn't always that easy. In my case, the mailserver was in a physical location where switching from a business-class static IP without vanity reverse DNS to one with vanity reverse DNS would have cost 10x the money.
If a company can't justify the money to spend on at least getting a dedicated SDSL circuit or fractional T1, then they really need to rethink their IT strategy.
That's the thinking that caused the dot com crash -- "Let's spend $1,000 a month instead of $100 a month so we can subsidize poor anti-spam software." :)
All I keep hearing is the same thing over and over -- "This test blocks a lot of spam." Now that is great, but good anti-spam software can block just as much spam while allowing more legitimate E-mail through.
-Scott
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