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Re: Fun new policy at AOL
From: Iljitsch van Beijnum <iljitsch () muada com>
Date: Fri, 29 Aug 2003 18:32:24 +0200

On donderdag, aug 28, 2003, at 20:10 Europe/Amsterdam, Paul Vixie wrote:

Play with DNS MX records like QMTP does.

here are at least two problems with this approach.  one is that an mx
priority is a 16 bit unsigned integer, not like your example.  another
is that spammers do not follow the MX protocol, they deliberately dump
on higher cost relays in order to make the victim's own inbounds carry
more of the total workload of delivery.  (additionally, many hosts do
more spam filtering on their lower cost MX's than on their higher cost
(backup?) MX's, and the spammers know this, and take advantage of it.)

Yes, that's why I don't use my ISP's servers as MX for my domains anymore. Having fallback MXes that only queue the mail for a while don't provide any real benefits anyway.

But how about this: in addition to MX hosts, every domain also has one or more MO (mail originator) hosts. Mail servers then get to check the address of the SMTP server they're talking to against the DNS records for the domain in the sender's address. Then customers who use an email address under their ISP's domain have to use the ISP's relay, while people with their own (sub) domain get to use their own.

For AOL and the likes this would also help against spam as they can rate limit incoming mail from unknown domains. Spammers are forced to register new domains all the time in addition to having to find abusable IP addresses so hopefully life for them will be a little more miserable too.

(Could reuse MX for this if a new RR is too much hassle, but large ISPs don't use the same SMTP servers for incoming as for outgoing.)

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