As for them sending mail from DSL:
1. If the business-on-DSL can define a different PTR hostname, they escape the "subscriber PTR hostname" filtering that the big network operators and ISP are enabling...
Can you please name some of these "big network operators and ISPs"? I've been following this for years, and am only aware of AOL starting to do this yesterday. A few unknown companies block E-mail this way, and a lot use it as part of overall spam control, but until yesterday I'm not aware of a single company/ISP that I had previously heard of that blocks based on the reverse DNS entry.
2. If above is not available, then the business-on-DSL mail server just relays its outbound mail through the IP provider's SMTP gateway.
Both of these are free, and the second is always available, immediate and easy.
Why would the typical business absolutely insist on NOT relaying its outbound through the provider's SMTP gateway?
What's the point of having your own mailserver if you are just going to relay through your provider? For example, by using your provider's mailserver, you have no way to prove an E-mail was delivered to its intended destination. You would have to coordinate with the ISP any time outgoing E-mail doesn't work properly. And doing that is probably going to be nearly impossible with an ISP that won't allow vanity reverse DNS entries.
And, of course, AOL has so many other options that are not so draconian.
While everybody would like 100% precision, if you have a horrendous, expensive, unending, worsening problem that was costing you and your clients dearly, impacting your bottom line daily, and there is a 98% or 99%-accurate automatic solution, what would you do?
Len, this isn't a 98% or 99% solution. It's about a 20% solution, as most spammers relay through mailservers or proxies. If that 20% solution then blocks 2.5% of E-mail, that's blocking 1 E-mail for every 8 spams that are blocked (isn't that a 12.5% false positive ratio by your method? <G>).
And, if everybody starts doing this, the dumb spammers sending from their own computers will just start using the open relays and open proxies, decreasing the amount of spam that is caught for every legitimate E-mail that is blocked.
-Scott
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